Thought I'd chime in here too.
I have 7 mac wifi connected devices here - 2 x iPhone 5s (ios 8), new MacBook Pro Retina (10.10), Mac mini 2012 (10.9), MacBook Pro 2011 (10.9), iPhone 4 (iOS 7), iPad mini retina (iOS 8). Also have Aiport Extreme (WPA2 and latest firmware) that handles all the wifi in bridge mode to a cisco router (wifi disabled) that does the cable internet to my ISP. Also have an airport express that extends the airport network.
All my iOS 8 devices (2 x iPhone 5s and iPad) run like a friggin dog on wifi - but curiously, not on all apps. Web browsing etc, seriously, it's useless trying to download anything. Browse imgur? Forget it. How about some Reddit and a few cat gifs? No way. After 30 secs of trying to load, I give up. I'm getting no better than 53KB/s on the iOS devices via wifi - yet, I can get 485KB/s through a crappy 3G signal from my telco on the same device.
Let's say that again to be really, really clear - 53KB/s on iOS 8 devices. Best I could get was 61KB/s.
Airport Utility tells me my signal for the iOS 8 device is "excellent" (1.5 meters from Airport Extreme, same distance as MacBook Pro Retina): Data Rate: 135Mb/s, RSSI: -58 dBm, Mode: 802.11n (5 GHz), Speed: 53KB/s.
My Macbook Pro Retina at same distance: Data Rate: 450Mb/s, RSSI: -52 dBm, Mode: 802.11n (5 GHz), Speed: 385KB/s.
All other the macs download (from the same wifi) at around 385KB/s on average.
Yes, I have spent days, literally days, trying to solve this *&^$#. I've tried every 'solution' mentioned in here and elsewhere, including but not limited to, DNS, WPA, Channels, Frequencies, Network Reset, Disabling Some Settings, Location Settings, Airdrop, Airplane Mode, Hard Resets, Factory Reset, Back Restore and more apart from jailbreaking (which I'm pretty tempted to do at the moment). To be clear, I have also used the Cisco router as wifi and get the exact same speeds through that wifi too, so this is not a router issue as far as I'm concerned.
I even went so far as to turn off everything electrical here (that was possible) and had only one iPhone 5s iOS 8 connected to the Airport router - no change in download speeds. Tried another router, one device connected, same result. So interference is not an issue I feel, nor is network traffic.
That some people have found solutions, is great, I'm jealous and pretty frustrated. However, my personal and experienced opinion - there is still something very very wrong with ios 8.
My results? My macs and iPhone 4 run through the wifi no problem. iOS 8 devices? Rubbish, worse than terrible. I'm actually embarrassed for Apple really.
The moniker of "it just works" is technically true, the wifi works, it's just rubbish. This does actually effect my business through lost productivity and increased costs as a result. Sustainable?
I was patient through the early iOS 8 updates, but a line has to be drawn. Apple - are you listening?
Fix this.