This post has the recent (7.18.1) measurements with Bridge VLAN Filtering that I made on my hAP ac² (also compared to the setup with /interface ethernet switch), no WiFi interfaces though.
http://forum.mikrotik.com/t/hex-poe-bridge-mode-is-only-a-switch/182489/1
Bridging between two ports at wirespeed with Bridge VLAN Filtering uses about 13% CPU at the minimum clock speed (448MHz, router is capable of 896MHz). About half of a core is loaded.