Sorry about the long delay for a reply, I've kind of been out of the loop and just picked this game back up for running.
Are you sure the fps isn't capped to 60 (checking with 'cl_maxfps' ingame)? I'm not sure what the default is, but I run the game at 90. Also make sure you don't have v-sync enabled in your gpu driver settings.
I've also had some weird mouse troubles with this game unrelated to framerate, not sure exactly what causes them, but I try to make sure I don't have any unnecessary programs running in the background when I have the game open. The only thing that seems to certainly get rid of it is running the game with the unofficial patch and turning on raw input, but I haven't decided whether to allow using it or not just yet.
https://download.kingpin.info/kingpin/patches/unofficial/mh's_kingpin_patch/kingpinpatch_v8.zip Here's the unofficial patch if you're interested in messing around with it. Also fixes a few other things like not hiding your weapon model at FOVs higher than 90 and adding widescreen resolutions.