What processor do you have with how much RAM? How much free, clean, freshly defragmented space is on your hard drive?
What model of camcorder is your video coming from and what is the resolution and format of that video?
When you first added your video to your timeline, did you see a yellow orange "render" line above the clip? This indicates a mismatch between your project settings and video specs, which can cause poor and inefficient program performance.