As there is no native support for GIF encoding in the browsers you very much depend on JavaScript solutions such as the one you're using.
The only way to improve the encoding is to go through the source code and see if you can optimize parts of it such as utilizing typed arrays (if not), conform to engine specifics knowing what works well with the engine compilers (ie. things like using simulated types, full constructor, non-morphing objects etc.).
Another tip is to encode the GIF when the video is not playing (stepping it forward "manually" for each frame using currentPosition
and the event for it) and with an off-screen video element. This may leave some more resources for the browser and the engine to run the script faster (depending partly on the system and its support for hardware decoding video - there will still be an overhead updating the video on-screen which may or may not affect the encoding process).
My 2.5 cents..