I use an Apple Magic Mouse 2 a fair deal in tandem with a trackpad. I've recently experienced severe 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 also got a Magic Trackpad but never experienced slowness or stuttering when using that for input. This appears to be exclusive to the Magic Mouse.)
The first lesson: keep your mouse charged. Many reports of lousy 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 constantly 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 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 seeking can be found in your ~/Library/Preferences folder. You'll want to delete the following:
com.apple.AppleMultitouchMouse.plistcom.apple.driver.AppleBluetoothMultitouch.mouse.plist
Reboot to rebuild them, and (hopefully!) enjoy a smooth and snappy magic mouse experience!
Are you still experiencing problems? (Updated ~fall 2019)
The fixes above prevent the lag 90% of the time, but I still experience issues from time to time. I continue to dig into this further and plan to update the article accordingly.
If you want updates, feel free to contact me directly, or you can share your e-mail address below to join a low-traffic no-spam announcement list that would include future discoveries around this.