To be honest I don't recall seeing this level of performance issue with monLocks on earlier than 15.7. However I don't have a 15.5 environment to hand to verify that.
I'm not sure if DBA Cockpit (use to monitor Business Suite on ASE) uses monLocks. With Business Suite, ASE is configured to disable lock escalation entirely so I would have expected they should be running with a huge number of locks given all tables are DRL locking. If so, I would have expected they would have noticed queries against monLocks taking an age.
It may be that they just monitor lock wait timing at an object level.