The Official Computer Tech Support Thread

[Freebase Dali]

@ Gunnels:

Try doing the test at different times of day, especially during late hours and see if you get the same result. Cable internet providers are notorious for oversubscribing nodes and you end up sharing a node's bandwidth with other subscribers on that node.
If that's the case, you'll notice you get higher speeds during lower usage periods, and vice versa.

If there's no change regardless of time-of-day, you may be getting throttled by Comcast due to high bandwidth usage. In both of these scenarios, you'll want to contact your ISP because this is an issue on their end and isn't something you can resolve without their intervention.

See what they tell you from their end, and if they can't offer an explanation then you'll have to troubleshoot things on your end. Contact them first, though, so you don't have to go through the trouble if it turns out they know what's going on off the bat.

P.S.
It's their responsibility to ensure that they're providing the services you're paying for, so don't let them try and weasel out of fixing the issue if it's on their end.