... and mp4 slows the processing down ...
compared to what?
YT accepts just a handfull of formats.. the YT-upload preset converts into the most useful - size/resolution/bitrate.
'speed' is defined by your ISPs bandwidth ...
You can reduce the file.size only by reducing quality = smaller resolution, lower frame rate, lower bit-rate ... is that what you want?
Converting imovie video to mp3
hmmm, mp3 is an audio-only format
Yeah, mp3 is just an audio format. If you you wanna speed up the uploading, two ways you can try:
1. Speed up the network
2. Reduce your video files - you can convert it to lower quality video with a converter while reset the video and audio parameters.