MatGetSubMatrix performance

[John Fettig]

What kind of performance should one expect with MatGetSubMatrix on a
MPIAIJ matrix, and is there anything that I need to know to get the
best performance?  Or is this routine best avoided?  I currently use
it, but find that performance varies widely from call to call.  One
time it will take 0.25 seconds, another time it will take 185 seconds,
and I can't figure out what would cause such a disparity.

John