I have one site with 300 MB and 575.000 (prefilled) posts, it is as fast as Yours.
The q2a performance is exceptional as long as one does not use pictures. Even if one downsizes the pictures through Your plugin or my extra code, it remains kind of slow.
A real big problem is the wysiwig editor, pages which do load the editor need too long to load, at least on 1und1.de shared hosting. On medialayer.com shared hosting even the wysiwig editor loads fast as light.