Latest Macbook here, opening files up to 10MB is fine but saving after making any change, even a minor one will cause Atom to hang for a few minutes. In contrast, Sublime has no issues. To be fair to Atom, it has no issues with actual code files in any language; it only struggles with log files and csv dumps. This limitation means I use Atom for most things and Sublime for the heavy files.
Most editors start to struggle at such file sizes, which is a shame, but try loading 100mb into Excel. The only editor I found to work nearly instantly is vi; nano takes almost a minute to load large files.