We had some performance problems due to the online users block.
The block was set to 30 min (instead of default 5 min), and about 300 students entered in a course to check their grades in a period of 1 hour (or less)... we had to hide the course block in order to use moodle.
We have now the block back to the default 5 min... but anyway we are not confident about it... and we are considering to hide it for all courses.
I wonder if this has happened to anyone else...
Online user s block is known to have performance issues. On bigger sites should be disabled.