I found the problem (for my installation): The »opengoo_application_logs« table growed up to 528,848 entries in a few days.
I exported that table an found out that any task was repeated more than 7,500 times(!!!).
e.g if i delete an email message there is one entry with action »trash« and 7,500+ entries with action »delete«. So OG makes more than 200 entries per minute or 3-4 entries per second.
I cleared that table and OG worked with normal performance. But after 30 minutes the table contained 37,000 entries again.