This isn’t a bug, per say, but something to look into. FluidProcessSlices slows down exponentially the larger the number of files being processed. 700 files…so fast! 4000 files…so slow 
This will be the size of the buffer rather than the number of files I would imagine, but we should test that because it would suggest different causes. The first possibility I’m already aware of. The second, not so much (but might be addressable from the language) .