As currently written, this plugin only allows one BGS to play at a time. (Soundtrack Layering is the more interesting plugin for this question, as it does allow multiple BGMs to play at once.) I haven't clocked performance, but have yet to notice any visual slowdown.
I have considered tweaking the plugin to allow multiple BGS-es, though.
Okay, using the F2 command, I see no FPS drops at all when loading four SE/BGSes at once in a loop, nor for four BGM layers in Soundtrack Layering, but four isn't exactly very many, is it? I suppose a real stress test of both plugins is in order at some point.