I have been using Apple Airport WiFi access points for the longest of time, and they used to work quite well. However, I found that their coverage were not always too great. If you have three stories or many walls between the access point and your device, the link quality will suffer greatly. The obvious solution is to add another airport and use it as a range extender. That works - to an extent. Bandwidth is reduced and the devices attached to the extender does not have near as reliable link as when connected to the main access point.

You can always hook up an ethernet cable to the second airport and put both on the same network, called a roaming network. But I never had terribly good experiences with this as it was hard to manage and know when you are connected to which base station (it is possible, but awkward as sometimes only MAC addresses show in Airport Utility).

Moving to two Unify AP-AC-Pro in mesh configuration solved all my issues. The best performance I have yet measured for a transfer from a device on ethernet to a WiFi enabled MacBook Pro is show below. That is using a file copy from the LAN machine to the WiFi machine using AFP. 1m22s to copy a 5.6GB file is not too shabby for WiFi.