How long (in minutes) is the video?

What codec was used to compress it?

What audio compression was used?

An hour+ H.264 video with .ac3 sudio may require many minutes to index the video portion, and many minutes more to conform the audio portion.

How long did you wait before trying to scrub the video after you imported it? If the answer is "not very long", try importing the video and adding it to a timeline, then go get a beer and watch some baseball for a while. Check back with Encore later to see if the performance improves any.

-Jeff