ok I did some more digging after getting the same issue with my ps4.
I found a few things out although I cant say this is 100% accurate as my info is based on what I read on other sites such as neogaf.
The first thing is the wifi interface on the ps4 is not very good, its 2.4ghz only and seems to struggle to even match comparable 2.4ghz devices, so if using wifi you probably wont get great speds anyway.
However with that in mind my initial speeds still were way below what 2.4ghz can do and seemed to be rate limited to 3-5mbit/sec. Neogaf users were suggesting after the 2.10 update speeds plummeted and I noticed people were reporting success by using their own isp's dns servers (my proxy didnt help). When I reverted to plusnet's dns servers (my isp) speeds improved a lot although still wasnt great, I now am probably hitting the 2.4 wifi performance ceiling.
So an educated guess is this, sony have throttled all the generic download servers and only local isp cache servers can get the unthrottled speeds. Sioo if using google or opendns switch back to BT's dns servers. If using wifi switch to wired.
This is only an educated guess tho.