Apart from what the others have said, more and more frequently I'm finding this is related to the client and not the WiFi network.
Make sure the clients are using the latest WiFi drivers.
Make sure the power saving on the clients is set to "Maximum Performance" for WiFi. Otherwise, machines crank the performance back. You can see this from the command line on Windows 10 with:
powercfg /SETDCVALUEINDEX SCHEME_CURRENT 19cbb8fa-5279-450e-9fac-8a3d5fedd0c1 12bbebe6-58d6-4636-95bb-3217ef867c1a 0
powercfg /SETACVALUEINDEX SCHEME_CURRENT 19cbb8fa-5279-450e-9fac-8a3d5fedd0c1 12bbebe6-58d6-4636-95bb-3217ef867c1a 0
Disable 2.4Ghz if at all possible. It's better to have one device that can no longer connect, than degrade the performance for every other user.