I recently purchased a couple of ISA bus video cards for an XT build, a Trident TVGA9000i-3 and a Western Digital WD90C00-JK, both 16 bit cards and with 512k Ram.
I thought I would see what their performance was like in my 486 machine with the Doom Timedemo, and compared with a TNT2 m64 PCI card on a Intel dx2-66, then AMD 5x86
at 133mhz then 160mhz. Options for overclocking the ISA bus are 1/4 PCI, 1/3 PCI. Amazingly the sound card and Video card had no issues even at 13.33mhz. It's blatantly
obvious that the ISA bus, at least in my Lucky Star LS-486e-D, is just too slow for 3D games like Doom.

The Trident and WD cards were almost identical so I'll just mention the Trident.

Intel dx2-66 results, Doom Timedemo1

TNT2 29.89 fps
Trident 8.6 fps ISA bus 8.25mhz
Trident 10.2 fps ISA bus 11mhz

AMD 5x86 133mhz
TNT2 47.4 fps

AMD 5x86 160mhz
TNT2 60.6 fps
Trident 11.68 fps ISA Bus 10mhz
Trident 13.55 fps ISA Bus 13.33 mhz