Is your BIOS configured to take the 133 mhz FSB of the Athlon XP 2100+? Is your processor overheating? How about BIOS settings like AGP fast writes, your Aperture size set to 128 or 256? How about your drivers? Are your AGP GART drivers installed? (Chipset drivers for motherboards usually auto install those, VIA 4 in 1's and the such.) and the latest GeForce 4 drivers? Like the 41.09 detonators?
There are many reasons for a slowdown, you should look into each one of these.