Comment 2 for bug 1415305

Took a look. It is using protobuf_bs.id() as perf report surface name which differentiates between surfaces within a client. So with multiwin, you get (0, 1, 2) :

[1422471924.629395] perf: 1: 61.44 FPS, render time 15.78ms, buffer lag 31.78ms (3 buffers)
[1422471924.630274] perf: 2: 61.44 FPS, render time 16.03ms, buffer lag 31.55ms (3 buffers)
[1422471924.631150] perf: 0: 62.31 FPS, render time 1.81ms, buffer lag 45.10ms (3 buffers)

It can be made to use MirSurface::name by passing it as an arg to make_{producer|consumer}_stream and then to BufferStream. Not sure if it's worth it, since it still identifies surfaces from a client.