Even my ~10 kb files syntax highlighting & scrolling lags on a computer where I play Battlefield 4 at almost highest settings.
Right now, Atom is just too slow when compared with other editors such as Sublime. Given enough time it shouldn't be an issue (maybe), but right now I think it's a huge pain point for a lot of devs.
In all fairness though: I wouldn't notice 99% of the time, and I can well imagine that this problem will disappear with time.
If atom has a daemon feature that I never found, or one upcoming, I'd consider it again. Until then my workflow involves too many client closes for me to not notice the significant delay that atom has when opening a new file.