Max,
First off, we definately need system specifications to see what your potential is.
Secondly, do you have command line parameters set? If not, try reading this thread:
phpbb/viewtopic.php?t=7126
There are many options, depending on your computer setup, so if you're unsure what you need, feel free to ask what is best for your machine (once you've posted your system specification).
Third, run a timetest and post the contents of the "fps.txt" file that is generated. To run a timetest, add the following commandline parameter to your D3 launch link (or to the misc tab in the D3 setup utility)
-timetest secret2.dem
Oh, and a final note, you should be using openGL as your video renderer unless there is some specific reason not to (ie: glide-enabled video card)