Unlike what one could imagine from intuition a higher antenna gain is not necessarily correlated with “better” or “faster” connection.
If norms are followed, there is a limit in the output power of the device (tramission), It depends on the device, but if the limit on a given channel is (say) 23 dBm, a Mikrotik device will subtract from It the antenna gain (let’s say 3 dBm) and the radio will output max 20 dBm, if the antenna gain is 6 dBm the radio will output 17 dBm, in order to respect the limit.
Things are different for reception, where a higher gain antenna may actually provide a better sensitivity BUT while It will be able to receive lower signals, It will also receive more noise/interferences, which might slow the actual data transfer rate.
To this you add that all these (indoor) devices antennas are omnidirectional, the shape of their emissione Is roughly that of a doughnut, this doughnut is “chubby” (smaller in diameter, but thicker) for low gain antennas and “skinny” (larger in diameter, but thinner) for high gain antennas.
Check this thread:
http://forum.mikrotik.com/t/does-size-of-antenna-matter/177500/1
The TLDR; is that radios and antennas are not simple to deal with.