When lots of clients are connected at the same time e.g. in a school, the AP does not immediately start to load balance them as it waits 30 seconds to evaluate each time. The setting is Spectrum Load Balancing Update Interval. Set this at a low level such as 2 seconds and the hand off should be really fast.
This setting is in RF Management > Radio Profile
Secondly, a problem in some places is that by default ARM is client aware and will not change channel if a client is connected. If there’s a device that is always connected this can cause channel issues with APs interfering with each other as they were not allowed to change. I would recommend changing this setting unless you’re using VoIP over your wireless.
This setting is in the ARM profile:
Here’s a link to the Aruba ARM Collateral. It’s good stuff.