I have some large HTML files. They are not normally viewed. I have noticed in one 3 MB file colourising disappears when I edit. This seems reasonable.
Another one is 1 MB. When I edit it, response to a keystroke takes several seconds. I presume the problem is the overhead of the colourising, perhaps re-laying out the whole file, not just the visible screenful.
Atom also keeps typing random bits of boilerplate HTML instead of my keystrokes. You might solve the problem by tuning the limit where to turn off colourising down below 1 MB.