It is certainly slower than Sublime, and is so on well-speced machines. I have machines with i7s, 16gb of ram, and an SSD, and atom still chokes when dealing with large amounts of data. Try multiple selections on large text files and watch it grind to a halt.

Certainly people are falling in love with Atom. It is open source ("hackable editor", as they say at Github), and of course this leads to a much quicker development cycle. Sublime has been criticized because features and updates are few and far between, and there appears to be a reluctance to open-source it.