While I appreciate all the suggestions above -- I truly do! -- I finally got my videos to load very quickly just by playing with the export settings in iMovie. In iMovieHD6, I was able to find the right the frame rate and bit rate by trial and error; in my case, exporting to Quicktime and using frame rates of 10 to 15 per second and setting the bit rate to 300 (with "optimized for streaming" selected) worked and allows my videos to start within a couple of seconds; as a final check, I went to the Apple store and tested my site on iPhone, iPad and Macbook, and the videos prepared this way loaded quickly on all devices. (In my case, 320 x 240 was a size that worked.) For videos that looked much crisper than the above and still loaded relatively quickly (though sometimes not as fast as above), the Mobile settings on iMovie08 worked. Also, a friend of mine had recommended Sorenson Squeeze as a fancy (and expensive) compression software option; they have a free trial (with watermark),and it seemed pretty easy to use, but after getting satisfactory results exporting through iMovie's Quicktime setting,buying additional software just wasn't necessary.