FreeBSD 9 seems to have some troubles with MySQL 5.5.20. A customer has moved from MySQL 5.0 on Linux to MySQL 5.5 on FreeBSD 9. He experienced a lot of periodic slow downs on the new, much stronger, system which he has not seen on the old Linux box.
This slow downs were also shown in high CPU system time but we could not see any I/O going on.
When we looked into MySQL we have seen many threads in
Opening tables state in the MySQL processlist.