Affects Version/s: 1.14.4, 20w29a, 1.16.2, 1.16.3, 1.16.4 Pre-release 1, 20w45a, 20w49a, 1.17.1
Fix Version/s: None
Whenever (long-lived) rising particles are confined by a block above (or inside) and an extended piston head to the West, the game's framerate drops sharply, easily to single-digit levels.
I originally encountered this when crushing large quantities of Blazes using sticky pistons and blocks, but I can also reproduce the lag using a row of campfires and extended normal pistons.
Steps to reproduce:
1. Build a row of pistons, at least 12 long, all of which will extend to the East. Place a line of redstone to extend them all at once, but leave them retracted for now.
2. 2 blocks east and 2 blocks down from the pistons, place a row of campfires with hay bales beneath them (so the smoke lasts longer)
3. 2 blocks east and 1 block up from the pistons, place a row of blocks which will block the smoke particles (e.g. stone or glass)
4. Extend the pistons and watch the framerate quickly drop to single digits.
5. Retract the pistons and watch the framerate instantly return to its previous level.
If the setup is rotated to face in any other direction, the lag is much less severe but is still noticeable in the Alt+F3 framerate graph.
This is quite bad with campfires, because the lag will be felt by any players within about 200 blocks regardless of the direction they are facing, and setting Particles to "Minimum" has no effect (unlike with Blazes, whose smoke will promptly disappear), making this a significant risk for griefing on servers.
Possibly related to
MC-124170, though that one was marked as Resolved in 1.14.2 while this one still happens in 1.14.4 (as well as in earlier 1.14 releases).
Does not occur in version 1.13.2 or earlier.