You mentioned physical size of monitors. Does it affect performance in your tests? I would be very, very surprised if performance on 27" 4k monitor was any different than on 42" 4k monitor. Maybe I'm not aware of some quirk?
At 42" 4k, there's probably no scaling, so more stuff fits on the screen.
Slight degradation on Dell 42" 4K, could be because of the ppi or the pixel ratio, which might be causing canvas/row/cell/ dimensions to be not rounded to whole numbers (this can affect perf).
We start of with the FPS monitoring tool which measure the frame rate, memory consumption and GPU sange
We switch to devtools->Performance analysis to measure function call times, layout thrashes and shifts, RAF bottleneck, memory consumption and CPU time.
We also use React devtools to measure re-rendering.
Scrolling is wrapped in Request animation frame loop, hence its being controlled by the browsers frame rate.