The bug

Pistons are no longer transparent when not extended. When triggering just a few pistons in a simple redstone circuit the light updates cause extreme client and server lag.

How to reproduce

1. Make a box / room from opaque, solid blocks so that no light can enter
2. Replace one block from the ceiling with a piston, pointing up or down
3. Toggle the piston state with a lever / button, for example

→  The client and the server will experience severe lag, beacuse of the excessive light updates

→  When multiple pistons with slime blocks are working in a redstone circuit, the performance is decreased drastically

Related to MC-130722

Analysis by adam279: Affects 17w49b, not the version before that.