Hi Patrick,
apologize for my late reply, I missed your reply.
Since you're on NFS - what version? - it would be a matter of poor performance in handling file locking, required by Moodle (by default is file based) .
Please, give https://moodle.org/mod/forum/discuss.php?d=398444#p1607452 and following posts a read.
HTH,
Matteo