I have now set up a post-commit hook that makes an
--incremental hotcopy. With the destination on the same
machine's HDD, it takes about two seconds, but with a
network share it lasts 30 seconds. Is it expected behavior
for committing a tiny change in a text file? If not, then
where shall I look for the possible performance problems? I
have svn 1.8.16.
It is probably due to slowness of the IO across network to read what is in the target repository and then copy over the files. Other than tuning NFS or whatever you are using there is not much you can do. This is why my first recommendation was to use svnsync. You could have a second backup server running and then use svnsync via https or svn protocol to that server. This basically replays the commit transaction so performs comparably to the original commit. It also makes it a lot easier to send the backup around the world or to another data center since it is using a protocol that is meant for that sort of latency.
That said, I have no idea what kind of performance you should be able to get via NFS. 30 seconds seems slower than it ought to have been.