I had the same issue, I was getting good response times up until about a week ago and it started being responsive in 'waves' sometimes great, other times it was like I was on a
56K modem.
I use windows 8 on a new HP desktop and have a
LAN cable connected to the modem/router I use.
I did a few things on my PC and nothing seemed to resolve this issue.
I updated the drivers (never mind that the PC is new and the drivers were working fine before last week), rebooted, same internet 'come and go' connectivity.
Power cycled my home network, and all devices, same thing. Same issue
Ran MS and HP updates, same issue.
Tried to disable my 3rd party AV and FW (Comodo and Antivir) and use the Win8 apps, same issue.
I then remembered that I had a DSL issue with my ISP about a week before, where connectivity was lost for some time.
So the other thing I did do, after I did the power cycle of my home network, is go into the command prompt and run a ipconfig /flushdns. and rebooted.
My issue seems to be resolved, my internet connectivity is fast and responsive again.
Not sure if this will work for everyone or if everyone should try it (or why I had to do this with Win8 when I never had to do it with WinXP) but it seems to have done the job for me.
Not sure if this is really what resolved it, but I would say it is something that takes about 5 seconds to do and I do not believe should mess with your PC settings.