AP handover on Mikrotik is super slow because ROS doesn’t support any roaming extensions to 802.11 protocol.
So clients basically do full reconnect on handover, this usually means full association and DHCP request etc. as if it was completely new network.
Some clients may be little bit more clever, but in general it’s slow.
And there is nothing you can do about it, no amount of tweaking of parameters will do anything.
Only thing that may help is to document your case and send it to Mikrotik support, so they know yet another customer wants roaming to work better.
But so far nothing have changed for decades!
Some previous threads about this topic:
http://forum.mikrotik.com/t/is-would-be-there-support-for-client-roaming-802-11k-802-11r-802-11v-802-11w/141353/1
http://forum.mikrotik.com/t/hap-ac2-cap-ac-roaming-is-a-joke/133251/1
http://forum.mikrotik.com/t/feature-request-capsmanager-roaming/78330/1 (2014!)
http://forum.mikrotik.com/t/wi-fi-roaming-802-11r-question/23184/1 (2008!!)