That's what I am wondering too, especially since it's a quad core, and Q3 can actually use more than a single core.. It ran fine on my 400 mhz pentium2 back in the olden times.
Maybe it's using software rendering? Try to run `glxinfo` and look for the OpenGL renderer string.
Quake 3 doesn’t have a software renderer. It requires OpenGL.
I mean, the system could be using Mesa with the llvmpipe driver. Software rendering but OpenGL 4.5 compliant. That's why I suggested looking at the output of glxinfo.