Anyone who has played Vivisector is aware that going up any sloped surface makes you run slower. This seems like a strange choice for what is supposed to be a fast-paced shooter, but it's simple enough to fix.
First, go to where you installed Vivisector, click on the Data folder, then Scripts, where you'll find "init.cfg". Open it, then look for "g.pspeedupaccel_limit". You'll see that it's set at 0.5, so just change it to 1.0 and you should be all done.