Magic mouse (1) lags singificantly when a USB C hub is connected (mac mini 2018, Mojave 10.14.5)

Have you tried different USB ports?

Log into another user account on your Mac and see if the problem persists. This will tell us if the problem is limited to your user account or is system wide.

Boot into Safe Mode and see if the problem persists there. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing.

This will tell us if there's any 3rd party software that's causing the problem

Lastly, if necessary, try the following:

How to reset the System Management Controller (SMC) on your Mac - Apple Support

Reset NVRAM or PRAM on your Mac - Apple Support

Poorly shielded USB-A - USB-C adapters are reported to cause wireless interference.

I have a SanDisk 128GB Ultra Dual Drive USB Type-C flash drive that is very handy and fast (170/40 MB/s sustained read/write with my setup).

But when I connect it to Mac mini 2018 (I use a short extension cable so I don't have to reach to the back of the Mac "System-S USB 3.0 Typ A (male) to USB 3.0 Typ A (female) data sync charger and extension cable 30 cm"):

The wireless Magic Mouse and Magic Keyboard start lagging and disconnecting. Moving the mouse closer to the Mac helps. Disconnecting the flash drive fixes the problem.

Other flash drives behave correctly so I think this SanDisk drive has poor shielding.

https://www.apple.com/shop/product/HLNB2ZM/A/sandisk-128gb-ultra-dual-drive-usb-type-c?fnode=5f&fs=f%3Dflashdrive%26fh%3D3783%252B45ec

https://www.amazon.de/gp/product/B00U2HVWK8/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

You can create a user account on your Mac as a trouble shooting tool. I have one that is a non-admin account for testing.

Yes, take it to the Apple store while you're still in warranty. Make an appointment at the Genius Bar for a free diagnostic

Be sure to make a full backup of your boot drive before taking it in, either with Time Machine or by cloning your boot drive. This is because Apple will probably want to erase the drive and reinstall the system from scratch to make sure there's no 3rd party software that might be involved in the problem.

I have a 2019 Macbook Pro with the same issue. Both Magic Mouse (1) and Wireless Keyboard (1) have issues staying connected. I upgraded to Wireless Keyboard Version 2 and now that stays connected, but my battery-operated Magic Mouse barely holds a connection anymore.

Based on the above reply about USB-A to USB-C adapters, I have a Satechi USB-C to USB-A hub which does not cause interference. However, when I plug my cheap-o USB-A 4-slot hub into the Satechi, Magic Mouse and Keyboard start lagging and/or dropping.

I have stable Magic Mouse (1) performance if my USB-A hub is NOT plugged in to my Satechi.