See this video by Teekeks for a demonstration of this issue.

Description

Redstone lamps turning on and off cause an unusually large amount of FPS lag when on/near chunk borders. While there is still some (expected) lag in the middle of a chunk, the amount on chunk borders is significantly higher.

Reproduction

1. Enable chunk border rendering with F3+G
2. Enable the lagometer and debug profiler by pressing Alt+Shift+F3.
3. In the middle of a chunk, build an 6x6 pad of redstone lamps, and place redstone dust on those lamps.
4. Connect those lamps to a fast clock (such as 2 repeaters pointing into each other)
5. Observe that there is a slight lag spike each time the lamps turn on.
6. Disconnect that system, and then build a similar 6x6 pad at the intersection of 4 chunk borders (such that there is a 3x3 area in each chunk)
7. Connect that system to a fast clock
8. Observe the much higher FPS drop

Notes

• The profiler node "scheduledExcecutables.checkLight.checkedPosition < toCheckCount" is the most time-consuming one when this issue is in effect.