We also experienced a slow down in running about 120 users simultaneously on multiple quizzes. We didn't have the stats switched on so couldn't tell too much except that page load times became unusable.
We are using some of Tim's OU question types (like drag and drop).
What we did see were database errors in the logs coming from processattempt.php
"Database transaction aborted automatically" processattempt.php
I would be interested to hear of any future issues and solutions if you have them.