Slow mouse movement only when browser is active

Solution: Go to Logitech "Logi Options" (the app that controls the features of the mouse and/or keyboard).

Upper right hand corner, select the "+" icon, then add Safari. Set the features on the mouse while in the Safari section of Logi-Options.

Works for other applications as well.

has there been a solution found?

This is a major issue for me

I had a very similar but opposite issue with my Logitech Lift (ergo) mouse. The pointer would move very slowly everywhere except on Safari. The problem was in Logi Options+ which allows you to have custom settings for the buttons and the speed of the pointer for different apps.

To determine whether added software is the problem, run in Safe Mode, where no third-party items are loaded.

"Works in Safe Mode, Fails in regular mode" implies, "it's something you added."

Start up your Mac in safe mode - Apple Support

Cursor Position Updates are performed at very low priority deep in the background. if you have too much computing going on, cursor position update will be wonky and slow.

What is most likely is that your Mac is overloaded doing needless busywork, and adding the browser simply loads it more, and sends it over the edge into un-responsive. The browser is not the cause, it is just the final straw.

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners. or a VPN that you installed yourself.

The idea that a third party, with no special knowledge of the inner workings of MacOS, can somehow find a simple way to protect your computer — that is not already being done by MacOS itself — suggests that the MacOS developers are somehow "holding out on you". That is absurd.

You should remove any and all (other than Apple built-in) virus scanners, speeder uppers, optimizers, cleaners, App deleters or VPN packages you installed yourself, or anything of that ilk.

Third-party file Sync-ers such as DropBox, BackBlaze, OneDrive, or GoogleDrive can ruin performance, but are not inherently dangerous.

Effective defenses against malware and ot… - Apple Community

<<. It is just as I choose the slowest option for the mouse acceleration. >>

if the slowest is unacceptable, why not choose a larger value for the acceleration? it can't be any worse, can it?

I've done this on several Macs and it works very well: https://paulminors.com/blog/how-to-speed-up-mouse-tracking-on-mac/

OK