Are they using the same amount of hardware acceleration?

If one has support for VP9 and one doesn't, then most of that is not because the cores changed in performance, it's because the video decoder got rearranged.

I have an early 2013 MBP and its still a champ, none of those problems.

I'd suggest popping off the bottom and cleaning out the dust, I do this about once a year.

Another consideration when making this kind of a comparison between an older laptop and a newer one is how much dust or debris are in each of them. If it’s an unequal amount, that’ll skew the spec comparison.

I don’t use Chrome much. Safari runs YouTube perfectly fine, even while running benchmarks for a dev project I work on at home.

Yeah, I understand Safari is easier on the CPU, unfortunately it doesn't allow for multiple profiles with separate histories/passwords. I also spend a bit of time on a Windows machine, and like having my history there too.