A surprising performance hit, bad performance with A-MPDU enabled. These are needed for high performance throughput, by avoiding a lot of overhead.
If most is default (no extra added mangle rules to set the priority) then all is priority 0 , and only the priority zero setting for A-MPDU is used, and that is enabled by default.
Mikrotik default configurations never set the priority.
Aggregation is good for throughput (reduced overhead), its bad for latency and jitter. So it should not be set for priority 6 and 7 (eg. voiceIP)
Expected performance without A-MPDU set: http://forum.mikrotik.com/t/how-to-get-rid-of-slow-wifi-can-it-be-done/139640/1