I mostly use Windows1252 encoded files with Atom, so I use it as my default encoding. When I open a big files (300kb for instance), sometimes Atom gives me that keep waiting message, and sometimes, I am forced to close Atom.
The opening is instantaneous, because the file is shown on a new tab, but as it always open it as UTF8, it still have to convert it to Windows 1252, and is that process that appears to be the problem.
Would you consider it a core or select encoding issue? Or maybe it would be threated by the new document package that will allows big files to be openned?