ISP throttling, otherwise known as just Throttling, is the intentional slowing down of the internet speed by your ISP (Internet Service Provider). There can be many reasons for ISP throttling, some legal and valid, some a bit sneaky.
A user can run a free speed test on Google and if the internet speed is substantially lower than what you have chosen for, your ISP is probably being throttled. However, Internet service providers can track your online activity and hence, can mold the result of speed tests to make you believe that you are being offered the speed you have been promised. Therefore, this technique is useless in most cases.
Read the full article at Tweak Library