Affects Version/s: Minecraft 19w04b
Fix Version/s: None
Campfires in 19w04b (and some earlier snapshots) cause extreme lag when there is a hayblock below and some other block some distance above them (for me it's down to 1-3 fps from the usual 50-60).
Retracting the block above the campfire or destroying the piston that keeps it in place instantly kills the lag, while just dousing the campfire with a water bucket only improves frame rate over time. Also for the setup and steps to reproduce, see image: