Preview plays back at an extremely slow framerate

Suddenly whenever I create a new project, the preview plays back at about 3 frames per second. It doesn't seem to affect old projects. I have my preview frame rate set correctly and set to skip 0 frames. I usually play back at quarter resolution but it doesn't seem to affect anything. My graphics drivers are up to date, and my Windows PC runs very well. This is stopping me from working on any projects, so would like an explanation.

Thanks!