So basically every particle is creating it's own surface?
Yep. A lot of sprites in B3D will slow things down quite fast, even if you've used CopyEntity. Is there a way to make seperate particles share a single surface?
Yes, you can create your own sprite system that creates lots of quads in one big mesh. Of course this isn't an easy thing to do (you also have to point all the quads at the camera yourself), but a few people have already created single surface sprite systems which you can use.