Not including outside factors than can contribute to slow performance such as power, I/Os, inadequate RAM, processor speed, graphic card or chip, OS errors, too many resource consuming programs opened at one time, or browser add-ons, are among the most common reasons found to cause slow disk performance are mechanical problems, a rouge program(s), or space.
A mechanical failure can contribute to slow performance. The only realistic way to fix this is to replace the drive. (Side note: SSD do not have moving parts but are still susceptible to data corruption.) Virus and Malware are another cause for slow disk performance. Antivirus programs will clean most viruses and trojans, but they will not touch the malware. Regardless of what the antiviral software claims, a malware removal technique is necessary. Running out of space on the hard drive can also cause slow disk performance. This is easily resolvable by archiving data to an external or network drive.