Pentium II 400 is much faster than a K6-2 450. Due to your CPU bottleneck, the GeForce card you are using with what I assume are much later drivers, is making the bottleneck even greater thanks to driver overhead. Did you try older drivers? If you had a Voodoo2, it might actually perform better than the GF2MX.
Lastly, don't compare user experience and playable framerates from back then with the sensibilities of today. I remember reading a review of Warcraft III from a local magazine and the reviewer was playing on a K6-III 450 (no joke) and was having a good time, describing it as playable, except for the final level, in which he had to send units to the enemy base blindly, because it was impossible to control them due to the lag.
I personally tried this on my own system some years ago after reading that review and finding it hilarious, I had the absolute best I could have had, K6-III+ clocked at 600MHz and lots of RAM that the reviewer did not have, plus a GeForce4 with very early drivers and I ran everything at 640x480 and everything on lowest to see how well it would perform. I can tell you now, it was never above 20FPS and it was usually in the teens and hitting single digits very often, even with just a few units on screen.
I personally would never play Warcraft III like that, but that reviewer from 18 years ago took no issue with it. The same might be true of DK2, which I have not played much. I know for certain though that I've seen it struggle even on fast Pentium 3 systems and what you're trying to play it on is more like a slow Pentium II.
