Bare letters do the heavy lifting. Archive (E), reply (R), forward (F), compose (C), and star (S) all work with no modifier, so the fastest way to burn through an inbox is single keystrokes, not chords.
Shift+letter often means "pop this out" or "the opposite action." Shift+R pops a reply out into its own window (versus R to reply inline); Shift+E marks a conversation not done (versus E to archive it). Once you notice the pattern it applies across most of the app.
Ctrl, not Command, switches accounts. Ctrl+1 through Ctrl+9 switches accounts, deliberately different from Cmd+1 through Cmd+9, which switches tabs.