1.In underwater many particles play and are very laggy.
2. The effect of lava sparks is also very low in fps.
And there are still a lot of things that show particle effects and they are not laggy in the previous snapshot and in 1.12.2.
3. If you breaking blocks the particles are lagging.
So the particles are very very laggy in this snapshot.
I hope I helped you fix this bug.

Sorry for the bad English. I am using google translator.