CPU speed of the Macbook Pro Retina downclocked
MacBooks do indeed tend to have some issues with their thermals. What I'd try to do in your case would be downloading an App to control your fan speed like Macs fan control. It's free, you don't have to buy the paid (donate) version. You can set your fans to spin up higher than the stock values Apple implements. 

Just for testing, you could set the fan to 100% at a CPU temperature of over 70°C and see if it does make a difference in performance.

I also do have a MacBook that's 5 years old now and while I don't do rendering or video conversion like you do on it, I had very high temps aswell just compiling code or browsing some heavy webpages with loads of JS. What I did was open up the laptop and replace the thermal paste. The stock paste that was applied was very badly (see this example from a MacBook Pro 2015). Sadly this seems to be fairly common and while these CPUs do sustain temperatures under 100°C pretty well, the OS often throttles it instead of running the fans on maximum.

Using TG Pro (just a personal preference, the tool linked above does the same thing) and reapplying my thermal paste I went from 60°C in idle to around 38°C and from 90-95°C to 80-85°C under heavy load.

That being said, the computer does get pretty noisy on maximum fan speed and reapplying thermal paste is not something everyone should just try. But the former should be worth a try.