Risk of Rain is known to have an issue with framerate - on some systems, the game won't get over ~50 fps, despite the system being more than powerful enough.

The fix: use an external frame-limiter. Options:

• The nVidia Control panel has one
• the frame limiter included in RTSS (part of MSI Afterburner) also works

I set the limiter to 120 fps, and get a constant 60 from the game.