Hello,
I'm still thinking problem is around database, because pages hang for a long while. Database queues queries and if there is a lot of activities on server, database can even crash.
For the moment, I escape compute() function from lib/grade/grade_item.php by returning TRUE. Computed fields aren't generated, but it works better.
There is already an opened bug report (#13426). I guess it's the same problem.
Anyway, thanks for helping.