iMovie, why so slow to do everything?

You have not posted the specs of the Mac on which you are running iMovie '11. What processor and how much RAM do you have installed.

While iMovie can import .MP4 files without encoding, if the .MP4 video file has not been encoded with an Apple friendly codec, then iMovie will need to re-encode the file. This is why importing the clips takes some time. Then iMovie will then need to re-encode the video output when you export. Video encoding is very processor and memory intensive. The time taken to encode on import and export is driven by the settings that you have for your project and Output file settings. So you might have highly compressed small files for import, but if you have widescreen HD as your project setting and you export a HD file then iMovie will try to upscale your source video assets. However as the detail is not in the original files then much of this effort is wasted.

Your Mac does not have a lot of memory and the CPU is getting on a bit now as well. I suspect the long encode time to export is driven by:

1. Small amount of memory (3GB no longer cuts the mustard as far as video editing is concerned).
2. Slow CPU
3. You export / project parameters in iMovie.

iMovie is seriously broken when it comes to importing video clips not in its native format. It can take hours, does hardly use the CPU and is not done in the background. So you cannot use iMovie while it is importing. The philosophy to convert everything before use practically makes it unusable. Once you try importing once you are unlikely to ever use iMovie again. However, a trick is to simply copy the mp4 files into to event directory of your choice (Movie / iMovie Events / <event name>). iMovie will then 'import' these files much faster when started. So it does actually support working with other formats without converting.