I guess it may be smaller updates like .NET and some drivers. .NET and most of the other smaller updates shouldn't even need restarting, they just eat your CPU and take ages because Microsoft is silently compiling them in the background on your computer.

Nowadays, Microsoft Defender's definitions are, for some reason, also shipped through Microsoft Update. You can get those daily or every few days, but they're fast and won't affect your system in any way.

Drivers usually shouldn't be common after installing the first batch, so I doubt you'd get a lot of those in 1.5 months.

Indeed, Microsoft only ships system updates on patch tuesday, the second tuesday of the month.

So this user could only have had a max of 2 system updates, 1-2 driver updates maybe (unlikely, though), and then possibly another 1-2 .NET updates. If the user has Defender activated, add a definitions update. Keep in mind that this is basically worst case scenario. Those updates should never cause issues, be that slow, or require 3 reboots.

Microsoft does only once a month sure, but OEM vendors push firmware, drivers, and BIOS updates through Windows update whenever they want. Windows is not the only thing that requires updating.