The bug
Particles cause frame rate to drop significantly, but apparently only when the game is not paused.
Ways to reproduce
Make sure particles are enabled
TNT
Place 5 blocks of TNT
Ignite one
Water
Go underwater
Barriers
Place about 32 barrier blocks down
Comparison
Side by side comparison of particles in 17w46a and 17w47b:
[media]
I am getting the worst lag when going underwater, pretty sure the particles are causing this, so I'll confirm same issue. Framerate will drop down as low as 4 FPS when I get a solid 60 on default settings above ground I did a check, and aboveground there seem to be around 150 particles, but as soon as I dive in water, they cap at around 3k particles. Switching Particles in Video Settings to Decreased will bring me to about 40 FPS underwater, and Minimal I see no drop at all. Windows 10 with a quad-core 3.1 Ghz i5, and a 550ti. Java version 8, update 151.