[kovidgoyal]

1) There's pretty much no difference. The only difference on linux is that various print statements in the code get enabled that print out diagnostic info to stdout

2) There should be no significant performance hit for most things. The occassional task might cause a lot of debug info to be printed to stdout that might be slowed down, but that should be rare.

The only thing I can think of is that in deug mode there is some extra error handling calibre injects into the main Qt event loop which might be working around whatever the underlying error is.