It's probably spotlight reindexing. This has been a problem with every version of osx for the past 20 years. Let it run overnight, and it should be caught up.
Came here to say this. This happens on all of Apple's operating systems - Mac, iOS, etc.
It's also a good idea to clean install all major (annual) versions of Apple OS's. When you install one major version over another, the upgrade script has to try to convert the configurations for most system services to the format for the new OS version and the possible combinations of settings in those configurations is virtually unlimited. A lot of post-upgrade issues and sluggishness result from non-optimal conversion of those configurations, especially on iOS. If you back up everything and format the drive before installing the new OS, everything will work much better. Yes it takes extra time to do that, but you will save many multiples of that time over the next year because your machine will run faster and have fewer issues.
I actually wouldn't recommend clean installing every single year (unless you're running betas). But for the person with the 2015 MacBook Pro, 5 years later is a good time to clean install.
It's also a good idea to clean install all major (annual) versions of Apple OS's. When you install one major version over another, the upgrade script has to try to convert the configurations for most system services to the format for the new OS version and the possible combinations of settings in those configurations is virtually unlimited. A lot of post-upgrade issues and sluggishness result from non-optimal conversion of those configurations, especially on iOS. If you back up everything and format the drive before installing the new OS, everything will work much better. Yes it takes extra time to do that, but you will save many multiples of that time over the next year because your machine will run faster and have fewer issues.