Mouse "randomly" slowing down or stuck in certain places (especially while starting a drag)

I just found this because I had the same problem. I noticed that the cursor would come to a crawl whenever I hovered across a list of links on a website. The culprit: I had my mouse cursor set to a larger size.

Solution: Go to System Preferences › Accessibility › Display, and set the cursor size to Normal (all the way to the left). After restarting Safari, the problem was gone. I can move across a list of links, the cursor switches between arrow and hand, without lag.

Since you can't seem to boot into Safe Mode, I would try either (or both) resetting the NVRAM and trashing the Magic Mouse .plist files.

Resetting the NVRAM

Your Mac uses Non-Volatile Random-Access Memory (NVRAM).

Here’s how to rest it on your model MBP:

1. Shut down your machine. Yes, a full shut down, not just logging out.
2. Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
3. Hold those keys down until your Mac reboots again and you here the startup chime.
4. Let go of the keys and let your Mac reboot normally.

Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Trash Magic Mouse .plist files

Since your issue persists both in and out of Safe Mode, I would trash the relevant Magic Mouse preference files.

As you don't specify what version of macOS you're using, I'll assume you're running a version from Mountain Lion and above. Follow these steps:

1. Go to Finder
2. Press and hold the option key
3. While keeping the option key down, click on the Go menu and select Library from the list
4. This should open up your user's Library folder in a new Finder window
5. Locate and open the Preferences folder (it's within the Library folder)
6. Within this folder find the following two .plist files:
   • com.apple.AppleMultitouchMouse.plist
   • com.apple.driver.AppleBluetoothMultitouch.mouse.plist
7. Now Trash these files
8. Restart your Mac

Your Mac will now create a new set of default preference files for your Magic Mouse.

Note: You will need to go back to System Preferences and reconfigure your Magic Mouse settings. However, before doing that I would test the Magic Mouse first to see if the problem behaviour still persists.

[EDIT - following successful Safe Mode boot]

Since you've now been able to boot into Safe Mode and the problem wasn't present, the first thing to do is to startup normally and see if the problem is still there. Sometimes just the act of booting into Safe Mode will resolve the issue.

If the problem is still there, the first thing I'd check is your login items. Login items can be incompatible sometimes with other software/hardware.

To do this:

1. Go to Apple > System Preferences
2. Click on Users & Groups
3. In the lefthand pane, select your user name
4. Now select the Login Items tab (near top right)
5. Take a screenshot of the list that appears (ShiftCommand4 and select the list with your mouse cursor) or just write down the list
6. Now click on the first item in the list so it's highlighted (i.e. click on the name, not the checkbox)
7. Press and hold the Shift key while you click on the last item in the list
8. All items should be selected now.
9. Let go of the Shift key
10. Click on the minus - button (at the bottom) to remove all items
11. Exit System Preferences
12. Restart your Mac

Once your Mac has booted up, test to see if your mouse is now working properly:

• If your mouse works fine, then start to add the items back one by one (using your list or screenshot). However, only add one item at a time and restart your Mac after each one. This process will help you identify which item was causing your mouse problems. Once you know which item it is, then remove that again from your Login Items.
• If your mouse doesn't work fine, let us know.