The wifi is:
Card Type: AirPort Extreme (0x14E4, 0x13B)
Firmware Version: Broadcom BCM43xx 1.0 (7.77.111.1 AirPortDriverBrcmNIC-1669)
MAC Address: 14:bd:61:f1:9e:f0
Locale: FCC
Country Code: DE
Supported PHY Modes: 802.11 a/b/g/n/ac
Supported Channels: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 36, 40, 44, 48, 52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140, 149, 153, 157, 161
Wake On Wireless: Supported
AirDrop: Supported
AirDrop Channel: 44
Auto Unlock: Supported
Status: Connected
I do not think it is a computer issue. If it was then all wifi traffic would be extremely slow, not just internal LAN transfers.
MM1 (Mac Mini) to MM2 wireless transfers 100KB/sec
MM1 to MM2 wired (over ethernet) transfers 100MB/sec
MM1 to internet 10-15MB/sec and sometimes more.
I did another test using my MacBook Pro and I see the same behavior, very slow LAN transfers over wifi.