I've had issues where the Wi-Fi seems to slow to a crawl sporadically. This only happens on my home network; I've not seen it happen anywhere else. No other devices are affected while this is happening and I can fix it by restarting the machine.
I know there are issues with Wi-Fi after upgrading to Yosemite, but this was brand new with Yosemite already installed. The Wi-Fi settings were imported through iCloud if that makes any difference.