try this.
start BL in single player mode.
load a build - like the afgan DM map you were having problems with.
open console ~ key
type:
metrics(fps);
close console ~ key
in the upper right corner of your screen will be a number - thats your FPS or frames per second.
now - run around the map, spawn a RL, blow stuff up, etc, etc.
While you do this, whats what your FPS number says
what do you get?
The bigger the number the better. Anything under 10 will be bad.
If you have good numbers -- more than 30, then your video is fine, and most likely the problem is on the other server.
if your fps numbers are low -- less than 15, then the problem is on your PC.