I am on 10.15.6 and this is very minor but still noticeable on my 16 enough to where I usually am using the dGPU. It happens with both mouse movements and typing, as well as UI animations (although to a lesser extent.) It is a very, very brief delay, but still pretty annoying. My assumption is it was worse with earlier versions of macOS based on what I've read, but I would have hoped this would have been eliminated a year later.
2.3GHz/32GB/5550 8GB
Edit - I do not think this is a flaw, but a feature, which is why it hasn't been 'fixed' (caveat am no computer guru). I think what I've read about the base clock frequency of the iGPU being low enough to induce UI lag when more processing is requested but a delay occurs makes sense, and I think this is done intentionally for battery life purposes (look at the battery life on laptops with very high resolution displays...most are between pretty bad to abysmal, even though many have large batteries as well). Disabling font smoothing reduces this issue (does not eliminate it)...to my understanding, doing this places more load on the GPU. Watching a video eliminates it temporarily...again placing more load on the GPU where we see increased iGPU activity. However, what I find eliminates it outright is actively being logged into more than one user account and simply locking one account while using the other. This makes it go away outright, or at least to a level I cannot detect. It also shows the iGPU as being more active, CPU temperatures are slightly elevated, and battery life is reduced. In other words, this seems to be increasing load on the iGPU substantially to a point where the micro stutter is gone, presumably preventing it from settling around that 350 MHz base frequency which becomes problematic when sitting at that base frequency and more graphics resources are rapidly requested (e.g., a rapid cursor movement, a UI animation such as Launchpad animation, or rapidly appearing text). Of course that low base frequency is helping to enable longer battery life...
I am guessing at some point a determination was made that this was the optimal amount of UI lag relative to getting battery performance of a desired level (i.e., more users would notice poor battery life than they would UI responsiveness if they skewed in the direction of better UI responsiveness on the iGPU.) All of the actions that reduce or eliminate the UI lag, even those that avoid reverting to the dGPU, also result in a hit to battery. Presumably the "fix" to this would be more user control over the iGPU settings, or perhaps even a mid-setting providing a modest amount of iGPU power efficiency between the most power efficient iGPU setting and the more power hungry dGPU, which is probably something Apple does not want to do. Nonetheless, it does leave me questioning why such a mid-iGPU setting would not at least exist for when on the iGPU and AC power, where energy efficiency isn't as important?
Unfortunately, my takeaway is that, in sum, you cant fix this issue with a big CPU, big screen, while maintaining a very high resolution and not seeing substantial reductions in efficiency. This is an intentional design decision.
