The OptiPlex 9020 has an SSD storage medium which is faster than most medium processor core personal computers that have been on the market for a similar time to it. Most medium processor core personal computers that have been available for around the same time as it have an HDD storage medium. Solid State Drives (SSD) store information in transistors on a circuit board rather than on optical or magnetic media. This has the advantage of being much faster than a traditional HDD drive.
An SSD storage medium is actually the best storage speed across all similar spec PCs that have been around for around the same time as the OptiPlex 9020.
An HDD storage medium is the worst storage speed across similar types of computer.