Well, there's several ways in general to switch a clock between different speeds, although probably all of them need more space than that one. So, the best way (or any possible way) to go about this kinda depends on how much room is available around there for it. One of the more understandable ways to do a varying speed clock is what zetlandsveryown said: Swap between two. Especially if you only need two options to toggle on a button. I think Roboticaust's old channel has a video demoing this from way back when. Also, I can point out one really general technique, as demo'd by my build (now on the second page of this forum): For lack of better terminology, enable and disable a delay slot along the wire. Basically, you could have a piston moving a block back and forth, connecting one 'path' along the wire when retracted and connecting the other when extended. You could use that to swap between 5 ticks worth of repeaters and 20 ticks, have all the repeaters on one side and 0 repeaters on the other side (so the delay purely adds time) or what have you. For a more complicated requirement, you could connect 1, 2, 4 etc. tick side tracks that can each turn on and off independently, so you can get any number accurate to 1 tick just by changing the settings. But for something like this, you could just have a... 6 tick delay on one part of the track, and then go through or skip the other part of the track (for the remainder of the time) with the toggle. I prefer to put the toggle on the input to a track, so you don't have the side track constantly activating when it isn't in use, tho with it on the input you may need to watch that the side track doesn't loop on its own and skip the first part.
If you want one side to be as short as the 6 ticks shown here, then for just about any side clock it might consume less space to use an entirely separate clock for the other speed. Tho most of the size required for a two clock system would be the TFF and toggle anyways, so it might save space to enable and disable a side track like that.
Not sure if this helps, since I don't really know how much space you have to work with.
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