Looking at other threads it sounds like Angry Birds (or more likely) game center could also be causing issues, as well as control center and then also airplay. Unbelievable oversights in QA testing. Is it even worth having betas when you don't listen anyway. Apparently wifi issues were brought up during all the betas but apple just released ahead anyway. iOS8 is a disgrace, the bugs outweight the gain. As far as I can see some worthless health apps (I'm not into all that health monitoring nonsense) versus countless bugs to exisiting functionality. Useless.
FWIW my throughput is all over the place. Slowing on speedtest to a quarter of expected speed and then out of nowhere picking up. It affects apps too where things that work intra network go sluggish, then fast, then sluggish with no ryhme or reason. I suspect airplay is a big factor on mine as I have apple tv and Airport express (used to join my wifi and provide airplay to my hifi). Half the time I will pull up control center and the airplay icon will be very slow to display. Sometimes it'll show only say the apple tv available and take another minute to show the Airport Express or vice versa. Very odd behaviour. The wifi potential is there as when it does work properly it flies along at old iOS7 pace but you just cannot predict what it's going to be like. All this on the iPhone 5S whilst my iPad third gen seems for the most part unaffected. Maybe down to different wifi chipsets in each device or the fact the iPad displays control center differently so maybe the implementation doesn't cause as many problems.