Thanks for replying.
I was planning to use AE for compositing and animation (like what Andrew Kramer does), but I tested it out with something I wanted to do straight away.
I think I made the comp size HD 720p, while the original video file was normal DVD size, 576i. And the file size was 5.32 GB.
I still don’t know why it was so laggy 🙁
Oh well, I just bought the Student/Teacher Production Premium, so let’s hope it works 😀