I created a flip splash of a sphere and saved it to disk (I tried Bgeo, ABC and OBJ). maybe 20 frames.

Then I made a pop network and that emits random particles on my road surface and used a for each with a copy to points.
I randomized size, rotation/orientation, and a retime set to particle age to play the geo seq off of the disk.
It's pretty fast with a couple thousand particles, but it's rain and I need 100x's that.
it goes REALLY slow.
Seems like the Instance node isn't really any faster.

If I bring the same geo into maya and use Maya's tragic instancer, it actually handles higher number WAY better.

How do I deal with large numbers of instance geo sequences?

I'm finding the same problem with other things like fur and grass.