There is a multi threaded xlib version written.
Do an archie search for mt-xlib:
Host export.lcs.mit.edu

    Location: /contrib
      DIRECTORY drwxr-xr-x        512  Jul 30 1992  mt-xlib
    Location: /contrib/mt-xlib-1.1
           FILE -rw-r--r--     106235  Jan 21 14:02  mt-xlib-xhib92.ps.Z
           FILE -rw-r--r--    1658123  Jan 21 14:03  mt-xlib.tar.Z
    Location: /contrib/mt-xlib
           FILE -rw-r--r--     106235  Jul 30 1992  mt-xlib-xhib92.ps.Z
           FILE -rw-r--r--    1925529  Jul 30 1992  mt-xlib.tar.Z