I wrote a blog post about using gulp to process all sass files as well as JS and images - removing them from the Jekyll work load at the same time as not needing to have Jekyll rebuild the site whenever a sass file or js file or image is edited.
Gulp will compile sass files instantly more or less, so using live reload, saving a change to a sass file almost instantly refreshes the browser.
By using gulp for all of those assets has cut my build time for Jekyll down to half a second from about 6 seconds - and that is if jekyll even has to run - any sass, js or image changes don’t need jekyll at all. Course that site is pretty small, only about 50 pages.
I’ve only just finished working this out, may still be some problems with it, and I am not at all a Gulp expert so any suggestions are welcome. So far it seems to be working really well, but I have not done much with images or js. When something changes in the images folder my console scrolls a zillion blank lines between the start and finished messages for some reason, seems to work ok though.