20 Minute Movie (Urgent)

I have a twenty minute movie that I am wanting to export. The project is extremely slow when running however the rest of IMovie (And laptop) is fine.

When I export I choose to do it through quicktime and by choosing the best quality settings, also by putting h.624 as the type. I then choose HD 1280, 1080 16:9 as my dimensions. And then I export.

As soon as I export it says '3 hours remaining' which (3 hours later) bumps up to '5 hours remaining' which 5 hours later is on '5 hours remaining' which eventually just stops midway with no explenation and no file to be found.

Is there any way I can export my movie in the best quality possible (shot on 1080p camera) with a reasonable file size as well as time to export without it crashing? Please help I need this fixed by the weekend.

I am using IMovie11 with the latest update.