Capped speed?? Link speed won't go above 78 Mbps on FAP231E

I've got FAP231-E connected to FG-100F firewall.

Everything works fine but when I use wifi analyzer app to look at the link speed it never goes above 78 Mbps on 2,4 Ghz, it will fluctuate below that, but never above, like it's capped at that speed. Is this normal? I attached a picture of the FortiAP profile I'm using.

On 5 Ghz the link speed stays at max 180 Mbps.