The 15 minutes can be to get a logon screen.

Or if already logged in to switch screens.

That sounds more like a resource constraint problem on the server running the OTRS code than a database problem – OTRS is a relatively large application, and can be fairly demanding on application server resources. Try monitoring CPU, swap and memory utilization at 5 minute intervals on the OTRS server for a few days and see if there’s any correlation to the problem periods – if the OTRS server is thrashing, the database response is irrelevant. Eliminate that possibility first, and then start looking at database performance.