Reddit mentions of Arctic Silver 5 Thermal Compound (2 Pack)

Right, but you can scrape off the thermal pads (then use alcohol to clean it), and use something like this: https://www.amazon.com/Arctic-Silver-Thermal-Compound-Pack/dp/B00LTYPTSO/ref=sr_1_1?ie=UTF8&qid=1494952157&sr=8-1&keywords=thermal+paste

When I did testing, I used the stock pads, and thermal paste, and without. Ran for extended periods of time at 100% usage.

The slowing performance is thermal throttling, the CPU gets too hot and scales speed and power to cool.

It does sound like either your fan is faulty or the thermal paste s worn out, 2 likely scenarios, both easily remedied, when the system is running have a look at the fan, if it's spinning freely and fast it's probably fine and it's the thermal paste, if the fan is slow and seems jerky possibly the fan, if it's not spinning then it's dead.

If it's thermal paste your going to have to change it, if you have no expertise it's a little unnerving, but a simple task.

Reapplying thermal paste video - https://www.youtube.com/watch?v=-hNgFNH7zhQ

Some coolers on amazon, you can get them many places depending on location. - https://www.amazon.com/s?k=1150+cooler&rh=p_89%3ACooler+Master%7CIntel%7Cbe+quiet%21&dc&qid=1568711306&rnid=2528832011&ref=sr_nr_p_89_8

Some thermal paste - I use this personally - https://www.amazon.com/Arctic-Silver-Thermal-Compound-Pack/dp/B00LTYPTSO/ref=sr_1_3?keywords=thermal+paste&qid=1568711550&refinements=p_89%3AARCTIC%7CArctic+Silver+5&rnid=2528832011&sr=8-3