Reproducing: Go inside any chunk of any map on 1.12.2 or later. Place a POWERED Empty [Command:""] Repeating Command Block AND connect to it 63 Empty Chain Command Blocks inside that SINGLE chunk.
Change a single of ANY of the commands to have an unsuccessful output (such as [Command:"Hi World"]), and the chunk will become affected.
OR you can also add a 64th empty Chain Command Block, and the chunk will also become affected.
If you have any 64 powered Chain Command Blocks, the chunk will become affected, even if all 65 Command Blocks are empty.
You can also have two Repeating Command Blocks activating 32 Chain Command Blocks each, and so on.
Conditional Chain Command Blocks don't seem to affect that count, you can have as many of them anywhere on the chain.

Whenever I LOOK directly at the Command Blocks that are affecting a chunk, I have big performance issues. Turning around and looking at them can cause a FPS drop from an average of 500 frames to an average of 100 frames, with no entities near.
Also, when I'm near those Command Blocks, I sometimes get TPS lag (visual rollback after breaking or placing blocks, world takes longer to load, player hand flickers while loading world) and also "ghost FPS lag", as my screen stutters even at 300 FPS, as if I was at 30FPS instead.
Have in mind that if I have 62 Chain Command Blocks, for example, they won't cause any noticeable lag. As soon as I place the 63rd/64th one, the issues above start happening, so it's not caused by the command amount itself, but by the bug caused from the conditions above.

Also causes entity flickering after teleports, until the entity is "refreshed" (likely by relogging or teleporting again).
A wolf is a good example. Tame a wolf and teleport towards the affected chunk using a button. The world should take longer than usual to load, your hand will also flicker during loading and after loaded the wolf that, due to his tp AI, was teleported with you will be flickering. You can still make him sit/stand, but he completely loses his collision-hitbox (can't be pushed around), and if he takes damage he becomes red until refreshed and makes an even weirder pose.

Affected Chunks:
Structure Block UI will not work properly anywhere inside affected chunks. After clicking the "Change Structure Block Mode Button", your screen will flicker and redirect you to the last screen (button will not work fully, but if you press done and reopen, you might or might not be at the next screen, based on which one you were).
Additionally, if you try to go from "LOAD" to "CORNER" mode, you will get stuck between both, on a new screen, that says "CORNER" mode, but has the "LOAD" mode's text fields. (see screenshot)
All buttons except "MODE" button in all screens will disappear after you click the "MODE" button until you reopen the UI.

Entity Flicker Example: https://www.youtube.com/watch?v=_NChbKy7kvY