I found the slow Wi-Fi issues may come from its Wi-Fi Signal/Noise ratio of Mac mini 2012.
When I check the Wi-Fi status by iStumbler, only the new Mac mini's noise lebel were relatively high and its S/N ratios are less than 1.0 always (e.g. 68/38, 59/41 etc) in cotrast to the others (MacBook Pro, MacBook Air's lebel: 66/13, 56/14 etc). The new Mac mini's sensitivity may higher but the specificity seems lower than the others.
Pls check the environment of your Wi-Fi networks' S/N ratio by iStumbler on different Mac.
iStumbler Release 99
http://www.istumbler.net
I guess this issue comes from the Mac mini's new Wi-Fi tip (intel's new tip?) with high sensitive antena installation.
I'm not sure this sensitivity and specificity issue can be controlled by software.
I hope Apple's engineers can sort out this issue.
Does anyone know this solution?