The problems on your Mac Mini include it being slow on startup and the processes mds_stores and mdworker spiking high CPU. These two processes are part of Spotlight, and if Spotlight attempts indexing the Mac hard drive and external hard drive with too much content while starting the Mac, it slows down the startup.
Generally, all Mac full startup time is in the 25 to 35 seconds range. Your Mac Mini took 15min to start which is not normal, the mds_stores and mdworker processes trigger a high CPU, and the problem disappeared while booting in Safe Mode, then the causes could be related to software that loads as your Mac starts up.
There are some proven solutions you can apply:
- Disconnect external storage media
- Uninstall third-party apps that use mds_stores
- Exclude items from Spotlight indexing
- Rebuild the Spotlight index
- Disable and reenable Spotlight indexing