You are replying out of context: if your file is on a network drive it doesn't matter if you have shiny UI, or text based terminal, you gonna wait for the round trip with your UI being unresponsive
You cannot allow editing - the existing buffer will be replaced once load completes. You can show the menu, but most options should be disabled. And it will be pretty confusing for user to see existing file remain in read-only mode after "open" command.
The most common solution if you expect loads to be slow is a modal status box which blocks entire UI, but maybe shows progress + cancel button. This definitely helps, but also a lot of extra code, which may not be warranted if usual loads are very fast.
If you pick another single-window app, my response is: that is a decision they chose to make. They can also choose to go multi-window or tabbed just like notepad did.