What stack of SolMan do you have installed?
Also have you install the central correction note and kept it up to date? Have you installed any of the MAI correction notes and kept them up to date?
There is a note specific to SolMan having its rdisp/max_wprun_time set to 1800 (or higher don't recall the value), don't recall the note but I bet it can be found by the parameter name and a filter on sv-smg.
I have SP 12 installed and only have 15 Dialog/30 Background and my system runs smooth; and has 50 to 75 systems being monitored.