Is that atom's fault or just the Julia REPL's fault? I use the REPL directly on Windows and it seems to be really slow as it will take something like "using JuMP" and precompile the module which takes time.
I just looked through Julia's settings tab in Atom, and saw the option "Fallback Renderer" with the note "Enable this if you're experiencing slowdowns in the built-in terminals." It was disabled by default, so I've just enabled it.
Subjectively, I think it feels fine now. Longer use will tell, but I suspect I was just running into a known issue some setups run into, and they already provided the workaround.
EDIT: Comparing running some code in Atom's terminal and a REPL running in the GNOME Terminal, the regular REPL still feels notably snappier -- even though I'm `using OhMyREPL`, which makes the REPL a bit less responsive.
I'd say Atom feels acceptable (and definitely not "mind boggling laggy" right now), and shift/ctrl + enter more convenient than switching tabs. So I will stick with it (for Julia). More time shall tell.