What would happen here in Minecraft?

What would happen if I built something like this?

20 command blocks each say hi 20 times a second, is that performance hungry?

It depends on how long you let it run. But should normally eat next to nothing.

Try it.

I would run it for 10-20 seconds. I've tested it, but I have a good PC that doesn't itch, but what about worse ones?

Shouldn't itch either.

All right, thanks for the answer!

Make 200 blocks and let them say hail 50 times a second. Then this is a great server crash machine.