The 2MB/slowness isn't something that can just be "coded away". It's a direct result of the technology stack chosen. It is not a solvable problem. Therefore Atom cannot succeed beyond niche usage.
The editor is intentionally crippled.
I'm the author of the Zeus editor and I regularly use an old 1.25 Pentium with 512 MBytes of RAM to test the performance of the editor.
Even running on that 15 year old machine Zeus stays fully responsive while editing a 60+ MByte file.
For a modern day tool a 2 Mega file limit seems rather low.
I'm guessing that Zeus isn't based on a web browser but was designed from the beginning to be an editor.
So it too is limited by the total memory available to the application.
For a 32 bit Windows application the available memory comes in at around 2 Gigs and for a 64 bit Windows application that explodes to 8 Terra bytes.
So even if Atom was 32 bit it should have 2 Gigs of memory to play with.
I'm not sure why Atom does limit files to 2 megs, but if it is because of the expansion of memory then there must be some serious expanding happening there.
But Atom was also supposed to be an editor...