Make sure that the GPU is actually being used.  This is an older card,
but it should render things faster than that.  "lsmod | grep radeon"
should show that radeon's been loaded.

  Look through the output of
glxinfo.  There should be a "direct rendering: yes"

and "Accelerated: yes"

and probably an "OpenGL renderer string: AMD OLAND (DRM 2.50.0,
4.17.4, LLVM 6.0.1)"