Problem: Updating redstone-triggered blocks with a signal causes all entities above it to recheck their state.
Reproduction Steps:
1. Shoot some arrows on top of a repeater or other redstone-triggered block.
2. Hook that redstone-triggered block up to a clock and hear the arrow fall and land again.

This seems to affect not only arrows, but items too. They will stack much quicker than normally, which is either good or bad, you decide.
It also seems to severely affect Client and Server performance after 30 or more arrows are on the redstone-triggered block, dropping TPS to about 5 and FPS on clients to about 20% of what it was before.