Some tips and notes regarding wireless speeds and why you'll be unlikely to get full expected rates on a testing service such as speedtest.net...
If your access points are PoE powered check that you're using gigabit PoE injectors or switches. The 24V injectors that came with older Unifi units are only 100mbps.
If you're connecting via a guest network ensure that there are no bandwidth limitations on those users.
There will always be a drop in speed on the wireless test results from speedtest.net. This is the nature of wireless and the results when using tools such as speed test. There are many factors to take into account including signal strength, SNR, distance from AP, shared bandwidth and the fact that wireless is half-duplex. For example, a device connected using 802.11ac that has very low signal would expect much lower throughput than one with better quality signal. The ideal signal strength range is above -60 dBm and below -40 dBm.
To get maximum "advertised" speeds with 5GHz AC you need to widen the channel bandwidth to 80 or 160MHz but be aware that that also makes the connection subject to higher noise/interference and at 160MHz will overlap neighbouring channels.
This is an older article but gives a good explanation of wireless speeds: https://www.speedguide.net/faq/what-is-the-actual-real-life-speed-of-wireless-374
And another, refer to point 2: https://www.tanaza.com/blog/5-things-know-wi-fi-speed-distance-covered/
If you use Ubiquiti's Unifi range and controller you can see transmit and receive speeds of each client device by enabling the TX and RX rate columns in your clients list view to get an idea of the actual connection speeds being established.