Andreiul

yes, that's an unstable overclock. This is far more prominent on Ryzen 3rd gen for example, which the chip ditch some work cycles in an attempt to not crash. Turing GPUs dont do this nearly to the extent of Zen 2 though
 
It's not overheating, but ever since Maxwell GPUs they scale with temperature significantly, in which higher temps reduce stability. It's reacting to the rising temperature by choosing a lower voltage point in the voltage to frequency graph, which usually means a lower frequency as a result.
 
PCIe 2.0 doesnt bottleneck a 1660ti, it's nowhere near fast enough to do that.

No. 

Worth? No, not really. You lose all sorts of monitoring capabilities, you introduce multiple points of instability and all the effort and hassle of potentially messing up something for a 100-200MHz clock increase.

From the last screenshot, you started at 3.22pm and at the time of the screenshot 16 mins later you've downloaded 7.3GB = average 7.6MB/s or 60Mbit/s, which isn't bad per say, just not close to your max speed.
As above, I would try a VPN... I get pretty much my max speed through a VPN, but haven't tested without it... also my max is only around 67Mbit/s (or 7.9 or so MB/s).

After doing some research:
 
https://community.digi.com.my/t5/Postpaid/Infinite-150-plan-throttle-after-one-minute-of-usage-Repost/td-p/47970
 
Not sure how you can directly tell if you're having your speeds throttled, but there's some evidence here that something isn't right. A VPN will protect you from ISP throttling, so that would be a good place to start. If you get a VPN and see your download speeds stay consistent then you are definitely being throttled.

Change the psu with a cx450 and i think you can do better in finding an ssd.