The reduction of stress on the body is mostly due to other factors, like the tires (and pressure) and the fork design. Possibly, the seatstays can affect comfort as well, but the main frame is a fairly stiff structure for “vertical compliance.” I doubt a lighter-weight main triangle is going to affect that much. Obviously, if you are comfortable, not only can you put out more power, but you also enjoy cycling much more.
However, I find fascinating it that even if you can “tough it out” on a super-stiff frame and super-hard tires, you aren’t faster, because they simply are less efficient.
I believe this is the blog post that was the inspiration for this cycling tips podcast, where you can listen to Jan on frame stiffness. I don't can't recally him talking about forks specifically though but I might listen to it again. While it matches my experience - I'll understand if it's not a line of inquiry people might wish to explore.