Maybe this has an easy answer,
I recently updated my site to use style sheets, but it was slow going because when I have the Html page and style sheet loaded, getting a preview takes minutes. I spend more time watching the spinning circle than working. This happens going from the code view to the preview whether or not there have been any changes.
My work around was to keep the styling on the HTML page until I was finished.
If I load a html page and not the style sheet for it then it works fast again. It seems having the page and it's style sheet open at the same time in the program causes this slowing and about 1/3 of the time crashing.
I am redoing my pages again so thought that I would try and solve this before I start.