A mechanical hard drive stores data on spinning platters read by a moving arm. A solid-state drive has no moving parts and reads any location at the same speed.
That single difference dominates everyday feel. Opening programs, booting, searching files and switching between applications all involve many small scattered reads, which is exactly where the mechanical arm loses.
For large sequential transfers — copying one big video file — the gap narrows. This is why a hard drive can still be a sensible place to keep archives and media.
Capacity per pound still favours mechanical drives, so the common arrangement is a solid-state drive for the system and programs, with a larger mechanical drive for bulk storage.