I think you use lag to describe the time it takes between something happing in reality and it being displayed on the display.
Assuming there is no initial delay for starting the imaging pipeline and display, this depends only on how long it takes to process and display a single frame.
In that case, higher frame rates are the way to reduce lag, not increase it.
The faster you can process the image, the faster you can display it, given an appropriate display. And that reduces lag. At 60 Hz for example, each image is displayed for 16.7 ms and the processing time can not be higher than that. At 120 Hz, each image is displayed only 8.3 ms, but processing must be twice as fast to allow such a frame rate.
Reducing lag and perceived blur this way does make tracking objects considerably easier though. I use a gaming display with a 144 Hz refresh rate on my computer and it is a night and day difference between that and 60 Hz.
The EOS R viewfinder has a resolution of only 1.280 x 960 pixels. It only does 120Hz 720p video, (1280 x 720 pixels) but I'm wondering: do we know the refresh rate of the R? I had assumed 60 Hz because that's what the LV display is, but the viewfinder spec isn't mentioned on Canon's site I believe.
In any case the M6 II does 120 Hz 1080p (1920 x 1080 pixels) video so it should have the processing power to drive a high res, high refresh viewfinder. A future high res R may give us just that.
Isn't the the resolution you named the resolution of the back-screen?
The R's viewfinder has a resolution of approximately 3,9 MP.