Why is my preview window so slow when I try to edit my project? Before you answer this question, you should know that all video editing software suffers from some degree of lag or frame drop – this is normal.
When a video plays in the preview window, it doesn’t play as a video on your computer or TV. In fact, it is uncompressed in real-time, and every second your processor and video card do millions of calculations.
The more video has been compressed, the harder it is to decompress it in real-time. That’s why the GoPro video can be very slow because it’s super-compressed. On the other hand, you can play very large uncompressed AVI videos that play very smoothly in the preview window.