The mesh throughput is not high. It is more designed for many to many, rather than simple point to point.
You could try disabling the 2.4Ghz radios (if the range is not too great) and running purely on 5Ghz.
And as @Adam says, directional antennas with give you more signal, and more signal gives you more throughput (or at least, more stability).
I don't know the range or other requirements, but as an example, a narrow patch antenna will give a far more focused beam (which means you also have to have them lined up better as well).
https://meraki.cisco.com/products/wireless/antennas-power#MA-ANT-3-F5