Sorry for necro this topic, but as I see people still visit this for some guidance on how to set a classic skyblock world properly, and - why not - I've come with a better solution.
Let's start by going to Singleplayer/Create New World, name it however you want, I suggest "Skyblock World" tho.
Select "Game Mode: Creative"
Go to "More World Options..."
"Generate Structures: OFF"
"Allow Cheats: ON"
"Bonus Chest: OFF"
"World Type: Superflat"
Here you need to go to:
In the preset box above, you clear all and then write "0;0;0" (this will make only one layer of air blocks) and then select "Use this Preset", then select "Done", again "Done" and finally " Create New World"
Activate fly mode by hitting 2 times space otherwise you'll fall in void ( that should be pretty nasty for a start )
Here you'll be using:
/tp x y z (this will teleport you to anywhere you set the coordinates) I suggest to use /tp 0 128 0
/setworldspawn x y z (this will set a global spawn point to anywhere you set the coordinates) I suggest to use /setworldspawn 0 128 0
/setblock x y z bedrock (this will place an bedrock block to anywhere you set the coordinates) I suggest to use /setblock 0 125 0 bedrock
This will prevent any laggy aspect you all complained about.
And now you're pretty done, you have an skyblock default world with one bedrock block as a point from where you can start to be creative and make your desired island.
Now it's up to you how you want to make it look like.
I watched this on Youtube, but then the youtuber was showing that the bedrock has to be at Y:0, for the hardest challenges. Thanks for it, but this must work for everyone. He said that you have to go 2;0;1 for no lag. Same, when you get on the world, fly up to Y:100. Maybe Y:128, but you will have to chose. As soon you reach the choice you picked, type: /setblock ~0 ~0 ~0 minecraft:bedrock . As soon you typed the command, it will summon a bedrock block. No lag and simple.