slow processing of process output when it's huge

[Dan Nicolaescu]

I run by mistake vc-print-log at the top of the tree of a Linux git
tree.  After 15 minutes of running I gave up and killed it.  The command
does finish after a while, I left it running overnight.

For that particular tree, the "git log" output is 44MB, 1146946 lines.
The time to run the "git log" command is about 3.5 seconds.
The interesting thing is that when saving the "git log" output to
FILE.gitlog and doing C-x C-f FILE.gitlog takes only a 3-4 seconds, even
when enabling font-locking.

So the problem seems to occur when running "git log" from vc-print-log.

Not sure this is a very interesting test case, but maybe someone wants
to take a look at what could be wrong...