So I managed to finally resolve this problem, just now.
Apparently, it turns out my router [the Tenda Technologies one] (which I had bought new to replace the one I accidentally fried) actually did have traffic controlling abilities turned on by default.
The setting affected the speed, in order to supposedly offer better overall latency or at least that's what the help doc says, which is how it resulted in a stable 3.5Mbps rather than my maximum connection speed like I wanted.
I guess it was to control congestion or something, which my network doesn't have any of as far as I know.
So, once I turned that off, the latency did become slightly erratic, but I gained the other 12.5Mbps I was missing in TMN speedtests.
This had been bugging me for like months now, but finally I solved it.
