What speed MediaGXm are you using? Benchmark results for all GXm speeds are reported in the Ultimate 686 Benchmark Comparison. For example, the FPU ability of a GXm-266 is about that of a Pentium 133, while the ALU ability is similar to that of a Pentium 166. On the other hand, if you look at Quake (software mode), the GXm-266 equates to a Pentium 100. For Quake II, it is similar to a Pentium 120. The DOOM results for the GXm-266 are similar to a Pentium 200 or a Pentium MMX 166.
I found the integrated graphics to have horrendous quality, so I went with a PCI Banshee card. The integrated audio also sounded terrible, so I went with an ISA sound card. There is an ultra DMA driver, which works in Win98SE, but it was a pain to get it working. Ultimately, I use a PCI Ultra100 card.
There are some performance tweaks for the GXm. I have outlined them here, Register settings for various CPUs
One such tweak is enabling FP_FAST, which helps the FPU results. For example, DOS Quake scores went from 33.1 fps to 36.2 fps after enabling FP_FAST on the GXm.
Basically, the GXm has speeds similar to the Cyrix 5x86, but are about 10% slower on account of not having any L2 cache. What the GXm has going for it is MMX. What I found surprising was that they didn't bother to fix branch prediction, not even with a GXm at 266 MHz. Yet branch prediction works even on the 6x86-P90.
Plan your life wisely, you'll be dead before you know it.