If I remember correctly, the Cyrix 700 has the same processing of 400Mhz PII, and from the recent review of VisionTek Xtasy Everything(it uses the same GF2 MX400), it has about 35-40% CPU with Athlon XP 1800+, no wonder your system uses almost 100% CPU power.
BTW, I have tested AIW Radoen and GF2 MX400 based card side by side. The MPEG1/2 encoding performance of the latter is far behind that of the first one.