Test to see if items despawn inside your worldspawn chunks. Based on your video what I suggest is dropping some items then starting your game and get teleported, then stay out there even after the clock stops. Wait for more than 5 minutes to make sure the items could despawn. Only then teleport back and look for the items. If the items despawned then spawnchunks are not unloading and so there must be some other problem.
Look into wolves, or making a potion machine to feed you potions of harming once every 30-seconds while you have right-mouse held down to throw them automatically.
Just tested, yes 2 observers observing each other make a clock. The 2 outputs are out of phase and can be run together to the same output, but it doesnt seem to make a dropper faster. Its hard to tell if a dropper is at max speed or not.
A sticky piston with a slime block pushes a redstone block downward. The redstone block activates the dispensers and another downward facing piston to activate the floor below, and so on in a downward ripple of pistons/slimeblocks/redstone blocks.
Lighthouses are towers with elaborate lighting at the top, usually redstone lamps that are turned on and off to give the impression of a rotating light. If a tower is built with a taper or is slender all the way up that results in a cramped space for placing redstone.
Here is a scheme for controlling 3X3 lamp panels in a 5x5 area using sticky pistons with slime blocks to push a redstone block into position. Obsidian or other non-movable is block is necessary because of the slime blocks, and to prevent the sticky pistons from powering themselves from the redstone block 2 slimeblocks are used on each piston to keep a minimum distance. A sequence of 4 pics (not a video).
The repeaters powering the lamps are set to max delay to stop the lamps from flickering, which also reduces Minecraft lighting engine rendering lag.
The last pic shows how the pistons are controlled, just a loop of max delay repeaters.
You have been unclear, cagey even, about these items. For all I know they could all be sugar canes from massive reed farm and so all one item type.
If its a mob farm that periodically flushes everything into a fall trap, then you need to handle like 12 different item types. Bones, arrows, gunpowder, string, zombie flesh, plus the 7 things witches can drop. Why not have an iron farm nearby too if you are spending time AFKing, so add iron ingots and poppies. So even if you have 20 items per second if those are distributed over potentially 14 item types it really isn't too much for a single hopper to handle if there is a hopper reserved for each type. But I dont really know whats going on so I'm guessing.
For my flushing mob trap i use a hopper minecart under the drop floor to pick up the drops, so no water stream and the items are guaranteed to trickle through the sorter one at a time because that is how they come out of the minecart, one at a time.
I just reviewed Nathan Ryan's videos on items in water stream to refresh my memory. Videos 2 and 3 are the relevant ones here. Here is the link to the playlist: Design Fundamentals of Minecraft 1.9 Item Elevators
First, the reason items get stuck on the first hopper is because of the recessed hole in the top of the hopper. To avoid that there are two possbilities: align the items in the water stream so that they always are to the side and not flowing down the middle by curving the water stream around a corner; or put a partial block over the hopper to block the hole.
Items are 1/4 block high and the same 1/4 block wide. A block on top of a hopper needs to be 3/4 of a block or taller to have an item be caught by the water stream pushing the item, but it can't be a full block because a hopper won't pickup an item a full block above it. Partial blocks that can work then are: soul sand, grass blocks converted to path blocks, snow layers stacked to 7 thickness.
You may have a capacity problem here. Each hopper has a transfer rate of 2.5 items per second, 4 hoppers will get you up to 10 items per second. A sustained flow of items more than 10 items per second on average will eventually fill all the hoppers and items will collect at the end of the item stream and despawn. You say your item flow is not constant, so 4 may even be enough.
Hoppers always push out the item in the first slot before moving to the second slot, then the second before the third, etc. If those first slots never empty because the item stream keeps those slots topped up to 64 then you effectively only have 4 different types of items you can pull out of the the item stream after the other slots of the hoppers fill to full stacks. It would probably be best to set up as many different parallel hopper chains as there are types of items, and set them up like an over-flow protected item sorter.
I'll stop here for now, this post is long enough already.
Put a carpet or lower half slab (or also, soulsand or path blocks) over a hopper to prevent items from clogging up in the indent of the model at the first hopper, then you can use multiple hoppers in a line.
You can use pistons to divert the water flow onto alternative groups of hoppers, with the pistons controlled by a clock.
You can use minecart with hoppers to grab items out of the stream much faster than hoppers can, and with the minecart straddling two hoppers you can get double the rate of item uptake into the hopper chain.
Hoppers cant be sped up, if you need 20 items/second performance from hoppers you need parallel chains of hoppers.
If you gave more details on what the full scope of your situation is maybe there would a way found to work around your problem instead of defeating it directly. 20 items/second is a lot.
A resettable timer by its nature requires access to the thing or process being timed. For example, if the timer were based on a block dropping through cobweb then to reset it means to be able to pluck the block out of the web. For another example, for a hopper timer a reset means being able to reach in move all the blocks into a particular hopper (and faster than a hopper would move them itself or what is the point).
So the problem requires a timer which is a regularly clocked counter with accessible bits, and then resetting the bits to the start state resets the timer.
I wrote up a method to do that using pistonless linear feedback shift registers in the thread Redstone Only Timers.
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Test to see if items despawn inside your worldspawn chunks. Based on your video what I suggest is dropping some items then starting your game and get teleported, then stay out there even after the clock stops. Wait for more than 5 minutes to make sure the items could despawn. Only then teleport back and look for the items. If the items despawned then spawnchunks are not unloading and so there must be some other problem.
Also you might try crossposting your problem to reddit at https://www.reddit.com/r/technicalminecraft/
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Look into wolves, or making a potion machine to feed you potions of harming once every 30-seconds while you have right-mouse held down to throw them automatically.
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What about 2 observers pointing at each other?
Just tested, yes 2 observers observing each other make a clock. The 2 outputs are out of phase and can be run together to the same output, but it doesnt seem to make a dropper faster. Its hard to tell if a dropper is at max speed or not.
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Look up gnembon's "Fun Farms" series, ep. 6 . Oh heck, heres the link
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A sticky piston with a slime block pushes a redstone block downward. The redstone block activates the dispensers and another downward facing piston to activate the floor below, and so on in a downward ripple of pistons/slimeblocks/redstone blocks.
0
Lighthouses are towers with elaborate lighting at the top, usually redstone lamps that are turned on and off to give the impression of a rotating light. If a tower is built with a taper or is slender all the way up that results in a cramped space for placing redstone.
Here is a scheme for controlling 3X3 lamp panels in a 5x5 area using sticky pistons with slime blocks to push a redstone block into position. Obsidian or other non-movable is block is necessary because of the slime blocks, and to prevent the sticky pistons from powering themselves from the redstone block 2 slimeblocks are used on each piston to keep a minimum distance. A sequence of 4 pics (not a video).
The repeaters powering the lamps are set to max delay to stop the lamps from flickering, which also reduces Minecraft lighting engine rendering lag.
The last pic shows how the pistons are controlled, just a loop of max delay repeaters.
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Here is a compacted version, put one item in one of the hoppers.
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You have been unclear, cagey even, about these items. For all I know they could all be sugar canes from massive reed farm and so all one item type.
If its a mob farm that periodically flushes everything into a fall trap, then you need to handle like 12 different item types. Bones, arrows, gunpowder, string, zombie flesh, plus the 7 things witches can drop. Why not have an iron farm nearby too if you are spending time AFKing, so add iron ingots and poppies. So even if you have 20 items per second if those are distributed over potentially 14 item types it really isn't too much for a single hopper to handle if there is a hopper reserved for each type. But I dont really know whats going on so I'm guessing.
For my flushing mob trap i use a hopper minecart under the drop floor to pick up the drops, so no water stream and the items are guaranteed to trickle through the sorter one at a time because that is how they come out of the minecart, one at a time.
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But what Mumbo used was something like this:
This is with a wooden button on the side of one of the red granite blocks.
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I just reviewed Nathan Ryan's videos on items in water stream to refresh my memory. Videos 2 and 3 are the relevant ones here. Here is the link to the playlist: Design Fundamentals of Minecraft 1.9 Item Elevators
First, the reason items get stuck on the first hopper is because of the recessed hole in the top of the hopper. To avoid that there are two possbilities: align the items in the water stream so that they always are to the side and not flowing down the middle by curving the water stream around a corner; or put a partial block over the hopper to block the hole.
Items are 1/4 block high and the same 1/4 block wide. A block on top of a hopper needs to be 3/4 of a block or taller to have an item be caught by the water stream pushing the item, but it can't be a full block because a hopper won't pickup an item a full block above it. Partial blocks that can work then are: soul sand, grass blocks converted to path blocks, snow layers stacked to 7 thickness.
You may have a capacity problem here. Each hopper has a transfer rate of 2.5 items per second, 4 hoppers will get you up to 10 items per second. A sustained flow of items more than 10 items per second on average will eventually fill all the hoppers and items will collect at the end of the item stream and despawn. You say your item flow is not constant, so 4 may even be enough.
Hoppers always push out the item in the first slot before moving to the second slot, then the second before the third, etc. If those first slots never empty because the item stream keeps those slots topped up to 64 then you effectively only have 4 different types of items you can pull out of the the item stream after the other slots of the hoppers fill to full stacks. It would probably be best to set up as many different parallel hopper chains as there are types of items, and set them up like an over-flow protected item sorter.
I'll stop here for now, this post is long enough already.
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A string of ideas that may or may not be helpful:
Put a carpet or lower half slab (or also, soulsand or path blocks) over a hopper to prevent items from clogging up in the indent of the model at the first hopper, then you can use multiple hoppers in a line.
You can use pistons to divert the water flow onto alternative groups of hoppers, with the pistons controlled by a clock.
You can use minecart with hoppers to grab items out of the stream much faster than hoppers can, and with the minecart straddling two hoppers you can get double the rate of item uptake into the hopper chain.
Hoppers cant be sped up, if you need 20 items/second performance from hoppers you need parallel chains of hoppers.
If you gave more details on what the full scope of your situation is maybe there would a way found to work around your problem instead of defeating it directly. 20 items/second is a lot.
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Not rowboats.
And since minecraft boats do not even have oars but are powered magically the realism argument does not apply.
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So the problem requires a timer which is a regularly clocked counter with accessible bits, and then resetting the bits to the start state resets the timer.
I wrote up a method to do that using pistonless linear feedback shift registers in the thread Redstone Only Timers.
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1/3 is also pretty bad, worse than 50-50 chance.