Originally Posted by ilovejedd
If I remember correctly, Calibre converts all non-epub files to epub for viewing. Perhaps the behavior you're seeing is because of that?
I doubt it, wouldn't there be a very noticeable delay when viewing mobi, etc if that were true? Plus I found an article from a year ago claiming ebook-viewer could handle all the same types calibre can convert.
That did start me looking around, though. Under Preferences, Behavior, "Use internal view for:", you can check ZIP and then it does use it's own viewer. Much better.