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)
Update: as of 21w10a, on Fabulous mode, the game cannot handle rendering 48x48x48 structure blocks anymore like it was able to in 1.16 (at huge hit to fps).
instead, at 48x48x26, the game is lagging as bad as 48x48x48 in the past. At full 48x48x48, the game takes many multiple seconds to render a frame and in some cases, just freezes for me. The situation appears to be getting worse with structure block rendering