would this semi afk farm cause too much lag on the server or would it work at all

While this should work fine, and shouldn't really cause any lag, can I suggest an alternative design making use of observers that has a few advantages.
It's slightly smaller, and doesn't use a clock so there's no need to turn it off and on. It should be easier to use too
You'll need at least two of whatever you're growing to start with, the first one you plant doesn't actually get grown.

In case you can't tell the block at the back is an observer, the others are dispensers with bone meal.

@NutjobBob While this should work fine, and shouldn't really cause any lag, can I suggest an alternative design making use of observers that has a few advantages.
It's slightly smaller, and doesn't use a clock so there's no need to turn it off and on. It should be easier to use too
You'll need at least two of whatever you're growing to start with, the first one you plant doesn't actually get grown.

In case you can't tell the block at the back is an observer, the others are dispensers with bone meal.

so the observer tells it is growing and sets off the dispensers loop repeats while you keep planting and allowing the dark to harvest for you.... thanks @NutjobBob

The farm you linked @thegoldsmith does indeed work, however you need to add a bit of extra redstone to make sure that one dispenser doesn't empty faster than the others. It's the same one I use.

Hyperclocks used to cause a ton of lag. Was that fixed?

@Cellexya The farm you linked @thegoldsmith does indeed work, however you need to add a bit of extra redstone to make sure that one dispenser doesn't empty faster than the others. It's the same one I use.

can you put up a screenshot of the alteration?