We are also experiencing problems with course restore since we upgraded from 2.4.5 to 2.6.2.
Our situation is explained here:
https://moodle.org/mod/forum/discuss.php?d=267097
Basically what happens is that the moodle node running the restore rushes in server load. Apache processes stacks up and requests coming on to this node times out if the restore takes too long. When restore is done everything goes back to normal. This is not the case for all restores though, some of them tend to run through more quickly. I think this problem is triggered when there are many users logged in.
We are using a shared NFS for moodledata and now we are investigating memcache as application cache instead, to see if that can solve this problem. Before we do that I would like to try to set this in config.php:
Another thing we are looking into is inserting an administrative node where we can force all our teachers to do their course restoring.