I build a (not to complex website) for which I added additional breakpoints, for once page one additional breakpoints. For the homepage I added 3 breakpoints, so in total 5. I had also two master sections created on this page which used on every other page (new banner and EU cookie alert), as a result every page had these 5 breakpoints applied.
What I found is that this not only caused Editor X to become slow and sluggish, it also impacted the load time of the actual website.
I removed all the additional breakpoints, expect for the standard breakpoints on 1000px and 750px. Both the editor as the website became significantly faster.