particles are billboards, right? on my machine, i can spam 30k particles without going below 30fps, but if i'm using billboard nodes containing a single quad, the performance noticeably goes down even if i only attach ~200 billboards to my scene. using the same amount of 50 polygon trimeshes (locked) had a much smaller impact. is the billboard node evil?
Locking your scene turns off transforms (and other scenegraph traversals), and you're probably sharing displaylists if you are using the same model (or a small handful of them) for your 50 poly trimeshes. If you are trying to choose between billboarded quads and some simple (no shaders, etc.) 50 poly meshes, I'd go for the meshes.