The average BSC value is 4.5 to 5s. It means that within this BSC, some cell might actually have up to 8 or 9 seconds SDCCH MHT. Can you confirm ?
If that’s the case, you should check the split of SDCCH causes : do you see a lot of SMS ? SMS requires more time to be transmitted than a call setup.
Do you measure a lot of SDCCH drop ? When the radiolink between MS and BTS is broken, the time to detect a SDCCH drop is about 9 seconds.
During these 9 seconds, the SDCCH is still active. Therefore, if you see a lot of drops, it can also cause a long MHT.
Typical cell value ranges from 3 seconds to 5 seconds (and more if there are a lot of SMS’s)