well there are probably a few possible explanations.
Does the isp have some kind of download limits on their connections? Like you can download 20gb of stuff/month? Because if there is one then it's possible that you crossed that limit after which they usually slow down the connection.
The videos being stored in your temp files shouldn't slow down internet at all, it could slow down the general speed of the computer. But since it stores them on the hdd and not the ram, it shouldn't make a difference unless the hdd is full/close to being full. You could always check the amount of space left on the c: drive, and maybe defrag it. Though as I already said these won't really affect internet speed, just the general speed (like how fast windows works etc, you get the idea)