The ProBook 655 G1 has an SSD storage medium which is better than most notebook PCs of a similar type and age to it. Most many processor core portable PCs that were released more than two years ago have an HDD storage medium. Solid State Drives (SSD) store data in semiconductors on a circuit board rather than using magnetic or optical media. Solid State Drives have no moving parts which makes them very fast and reliable.
An SSD storage medium is actually the best storage medium amongst all portable PCs that have been on the market for a similar time to the ProBook 655 G1.
The worst storage medium across similar types of portable PC is HDD.