Glass and Trees primarily, even simple polycards for grass seem to TANK performance, even at low numbers. i converted each particle to individual objects. 80k particles with 320000verteses made blender literally freeze for over 10 minutes.
use children way faster !
The trick is to use less object.
You can for example make 3-5 leafs per textures on and object instead of one and then randomize it.
Also as Ricky said, use children and don’t convert to real object.