You are facing a tired drive issue. Over time the files on it (OS, Apps & data) get fragmented so it takes a bit of time sometimes for the OS to load as it needs to find all of the pieces which are scattered across your drives patters and tracks. The best thing here is to replace the drive given its age.
[guide|3113]
Make sure the drive you get is able to run at SATA II (3.0 Gb/s) many drives today are SATA III (6.0 Gb/s) and can’t run at SATA II (3.0 Gb/s) speed your system needs to run at. Review the drives spec sheet if it doesn’t state it don’t use it.