The normal operation of your PC should remain the same.
The HDD will be as slow as an HDD is when you use it.
Boot process may become slightly longer. But not significantly.if I add an old HDD (just for movies etc.) to my build which is with NVME and SSD, will the PC become slower?
The normal operation of your PC should remain the same.
The HDD will be as slow as an HDD is when you use it.