Poor WiFi Performamce - FortiWiFi 60E with FortiAP 421E

I have a FortiWiFi 60E that I am using (running 6.4.x) and found WiFi performance to be poor.  My WAN connection is capable of 500M and WiFi is barely 100M.  If I use the built in WiFi on the ISP supplied router then this reaches the full 500M easily, so I know it isn't a device limitation.  Have tried various devices as well and all get same results.

I had a spare FortiAP 421E so I hooked that up and get the same poor performance.

I have tweaked multiple settings and channels but can never get better than about 150M.  I have updated firmware on both 60E and 421E and no improvement.

What am i doing wrong and what do I need to focus on to get this performing as it should?  A cheap ISP WiFi Router should not be better than the Fortinet devices :)

EDIT: Forgot to add that I would like to keep the 421 attached if possible as I need dual band and I believe the 60E is single band only.