The only way to compress video is to re-encode it. You can encode with lower quality settings than the default video recorder, and decrease the resolution.
There are a few options for client-side encoding, but they are functionally limited.
Whammy.js
Encodes webm, but currently doesn't support audio. Bummer.
Firefogg
Encodes webm and ogg. Only works in firefox.
Encoding is a cpu-intensive process. It will eat a ton of battery, and will take a while to process. If your goal is to save time on the upload, this probably won't help you much.