The bug
After playing around with some toggle-text signs I found the game would drop from 300+ fps down to around 10 after a few toggles, which seemed very odd. Looking away from the sign would return the fps back up high. Believe it has something to do with extra extra: [] tags appearing.
Running data merge block <pos> on a sign will add an extra: [] to each Text part of a sign. This effect also seems to stack. See images below. First is the initial sign data. Then I add an empty text, then I have done it 3 more times.
I can confirm that the "extra: [] tags" can cause extreme lag. They are created not only by data merge, but also by copying them with "pick block", or by the clone command, or structure save / load.
This is especially bad for map makers, a map with only a few copied signs is unplayable.
The picture shows a sign before and after it was copied with "pick block".
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