Since updating my iMac 2013 to Yosemite the wifi transmit speed Tx is very slow. It wil initially show 1170Mbps to my Airport Extreme then it will fall to 195Mbps and it will fluctuate occasionally up to 1170Mbps for a second or two before falling back again. On mavericks wifi maintained a solid speed of between 700 and 850Mbps occasionally hitting over 1000Mbps but it never ever fell below 700Mbps. I've tried a transfer speed and its dropped from 40-50MB/Sec to between 8 and 20MB/Sec. I checked my rMBP from the same desk and its solid with no issues.