Seriously? Thermal-based throttling is a hardware feature, built into the processor, and it cannot be turned off. If the processor temperature reaches the Maximum Junction Temperature (typically 100c), processor performance is going to be throttled; clocked down until the temperature issue is alleviated. Again, this cannot be turned off; it is a nonmaskable hardware feature. The secondary problem is that this particular processor family does not support the PECI bus, so any external fan speed controller will be unable to access processor temperature readings and directly respond to changes (they could only indirectly do so based upon temperatures in other locations). There are ways to implement fan speed control at runtime using ACPI, but this is a feature typically only used in laptops where passive cooling and battery performance is also a consideration. Bottom line, if processor activity is sustained, it is possible for the processor to reach temperatures where throttling can take place and it is definitely something you need to look at. Run something that will provide you with a realtime display of temperatures. If they aren't reaching throttling levels, then great. Regardless, at least you will know.
As for the other possibilities, you indicated that no issues occurred while watching locally-stored content. That in itself points to an issue at the networking level. Are you in the U.S.A.? There was a report released this morning that indicated that the big four ISPs have been throttling Netflix and YouTube all the time - and were doing so even while Net Neutrality was still in effect.
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