Playback is incredibly slow when basic effects applied

If I change the speed of a clip or drag and drop a simple effect, the playback on that clip becomes nearly unwatchable and lags like a MF. Please help!

More details are needed.

Computer specs.
Project settings.
Media format.

If I change the speed of a clip or drag and drop a simple effect, the playback on that clip becomes nearly unwatchable and lags like a MF. Please help!

What specific effect? You throw enough hardware at Resolve, it can play quite a few effects in real time. When it doesn't, use the render cache for real-time playback. Some stuff may never be real-time because it's just too demanding -- I'd put Face Refinement, Magic Mask, and Depth Map in that category. Some people can run NR in real-time, some cannot; it's very demanding from a system hardware point of view.