I don't think being "browser based" as the reason that opening a directory is slow. A lot of traditional IDEs (cough Visual Studio cough) also have similar problems. I think they're doing a bunch of work (e.g. caching all subdirectories and files) but aren't doing it concurrently with the UI.

I feel we are comparing apple to oranges here: sublime (a text editor) will simply list files & directories in the top level folder when you add a folder to the projects while an IDE (thinking mostly of Eclipse here, but I assume other IDEs aren't that different) will parse every single file to build ASTs, symbol lookup tables and all these things that you either love or cannot stand.