could be many things such as:
isp having issues - call to get information on resolution time.
you have reached your fair use cap and are being restricted - call isp to unrestrict your line.
on wifi? interference from lights/environment/walls/signal strength/channel conflict. - try a hard reset of router / use rj45 (network cable)
local net same address as another ... usually you get a warning that youre possibly sharing a local ip address. - reset router (easiest)
possible adware/spyware, virus, trojan, browser hijack. - run av
using chrome. - stop using chrome.
or there is another computer on your net which is downstreaming at close to your provided speed. eg you have a 1mbit line, and some1 is streaming shit on their tablet at 0.9mb ... meaning you can surf, but downloads are taken by the 'demand' of the tablet (cause this is how andriod tablets trick you into thinking they are fantastic wifi devices).