Older drives are just slower(for the most part). As advances like PMR have made newer drives faster.
Remember that faster may be sequential or may be random. Random reads/writes are helped with a drive that is minimal fragments. So as the user above says, give that thing a defrag.
If the system is new enough to take a newer drive even a small one(120 gigs is already very small by modern drive standards) may be better(you can also keep all date near the start for better performance by trapping it in a small partition.).
It used to take 2 fairly good old 250 gigabytes in raid 0 to even remotely match some of today's slower drives in sequential read/writes.