Arte you certain your Zoom or whatever is using GPU?
I had to tweak something to make Zoom and Youtube see the GPU better (for Youtube, something in the Firefox about:config), and it lowered CPU load on my i5 CPU quite a bit, down to things becoming pretty fluid (T470, currently at 6.0.12 but it worked with 5.x, too).
While I fondly remember my T420 from 2011, its CPU was sort of... underpowered by today's standards.
I certainly tinkered around a lot with it over the years until it was blocking my work when I got my first remote job. Seeing the rebuttals here, sounds like it's worth dusting it off and giving it another shot at some point.
I had to tweak something to make Zoom and Youtube see the GPU better (for Youtube, something in the Firefox about:config), and it lowered CPU load on my i5 CPU quite a bit, down to things becoming pretty fluid (T470, currently at 6.0.12 but it worked with 5.x, too).
While I fondly remember my T420 from 2011, its CPU was sort of... underpowered by today's standards.