Wifi faster than gigabit ethernet?

I'm a little stumped at this one, so perhaps you can help me out?

Small office environment.

On Gigabit Ethernet: 5 VoIP phones, 7 PCs, 1 database server, 1 printer/copier, 1 Netgear AC router

On Wifi: Whatever miscellaneous cell phones people are walking around with.

Internet Service: 200/20 Cable

The VoIP phones are new, and we did change the QOS on the router to favor them, but WIRED performance to the PCs has fallen off a cliff. Various speed testing sites (Speedtest.net & Fast.com) report <5 mbps to the PCs. The phones on wifi report 125 mbps.

I originally blamed the VoIP phones overwhelming the very very old Powerconnect gigabit switch. I went with an unmanaged TPLink 48-port to give us some expandability, since the 24-port was nearly full. That helped some, but I'm still getting the very mediocre performance on the wired PCs.

Any idea what could be going on? I doubt it's the router, since the router is also the only wireless access point. The switch is brand new and should have sufficient switching capacity for this moderate load.

I'm not sure where to look next.