if you move the asset processing to Gulp or some other task runner you can speed stuff up quite a bit. Gulp can process sass almost instantly, so if you are just working on css then that is much easier - but it also removes that step from jekyll so it makes the rest of what jekyll does faster. Gulp can also move the images which is another thing that takes time for jekyll to do. I have my site so that jekyll mainly handles the page creation/layouts/includes. Gulp handles sass, js and images.
Check this post for info on what I did and also info from MM on how he does it. It can be a little complicated if you are not familiar with Gulp, which I was not, but I got the hang of it.