I've had the same problem for 2 week's now after an engineer was in the area to fix a problem, i've had an engineer out at my house and he replaced the router, ethernet cable and the cable coming into the house and still nothing has fixed it.
Although i noticed the problem's seem to be HTTP, i've used Steam and other gaming clients to update games and i hit much better speeds but downloading anything on sites, using any speedtest or even trying to watch netflix/youtube it will buffer quite abit.
We have tried everything on my end and it's not my problem, we used different computers, browsers, wifi, router mode, modem mode, my own router and different cables.
I was told i would get a call on monday but obviously that didn't happen, i'm in contract now but i'm not going to continue to pay for bad service
I downgraded my connection back to 200mbit then spent another few weeks trying to get the speed fixed which they finally did fix and was nothing todo with my end.
Skip forward a few months and today i was offered the 300mbit line again at a cheaper price and it was turned on earlier and right away my speed dropped to below 47mbit with most servers saying under 12mbit.
Upload seem's fine, ping has also doubled now aswell.