Question then becomes this: if GLAMOR is a frontend to OpenGL, what is GLAMOR doing that makes OpenGL slow down in RadeonSI? Is the issue in GLAMOR, or in one or more OpenGL commands that are still slow in RadeonSI?
1. Missing functions entirely. Tests like the lines and circles that are spectacularly bad happen because glamor doesn't accelerate them at all, because no one has written the code to do that in OpenGL yet.
2. Batching lots of small operations together. In lots of cases the 2d driver might run a bunch of commands together, while in OpenGL it tries to do stuff like binding a separate texture per character, which introduces a lot of overhead. I'm not sure how much of an issue this really is in practice - it definitely needs a lot of work to look good in benchmarks, but I don't know if this is something you'd really notice that much in practice or not.