When having a ton of Structure Blocks or a single one with a large size set, turning on Show Invisible Blocks mode for them will tank the fps down to potentially single digits. Even when using a smaller size for Structure Blocks, the performance impact is still very noticeable.

For the test below, I setup a superflat world with mob spawning turned off. Then I placed a single Structure Block and set the size of it to 48x48x48. Then I clicked Show Invisible Blocks and my fps goes from 350+ down to just 5fps.
Here's the streamable link as the video is too large to attach directly to this report:
https://streamable.com/ghygfp

Also attached below are pictures to show the issue if the video doesn't play.

I have tried looking to see if this was reported already and was unable to find another report. Just mark this as duplicate if there is another report but I am fairly certainly this issue went unreported for a very long time (I recall experiencing this issue in 1.15.2)