I use an Apple Magic Mouse 2 a fair deal in tandem with a trackpad. Recently I've experienced some serious sluggishness/lagging in Mojave (I'm not running Catalina yet), and I've had great luck with the following fixes.
Fingers crossed 🤞 this works well for others with similar wireless mouse problems. (I've got a Magic Trackpad as well, but never experienced slowness or stuttering when using that for input. This is exclusive to the Magic Mouse.)
The first lesson: keep your mouse charged. Many reports of bad behavior relate to the device having a lower battery. Charge regularly and there's a good chance you'll be off to the races again. (And re-charge yourself, too. Read something, or take that short walk-around-the-block you always tell yourself you're going to take! 🤜🤛)
If you use a mouse pad: make sure your hand position isn't keeping the mouse too near to the top edge of the mousepad. Passing over any edge/threshold (for some optical sensors) can create confusion and cursor stutter, though it really seems to stand out with the Magic Mouse.
And for the "under the hood" stuff: from a Lifewire article I came across, we've got:
...a number of preference files related to the mouse that could be causing the problem. As a result, you can [...] get rid of all of them, and let your Mac rebuild the preferences.
The culprits we're looking for are located in your
~/Library/Preferences folder. You'll want to delete the following:
Reboot to rebuild them, and (hopefully!) enjoy a smooth and snappy magic mouse experience!
Still experiencing problems? (Updated ~fall 2019)
The fixes above prevent the lag for 90% of the time, but I still experience issues from time to time. I'm continuing to dig into this further and will update the article accordingly.
If you want updates, feel free to reach out to me directly, or you can share your e-mail address below to join a very-low-traffic no-spam announcement list that would include future discoveries around this.