If you read data to an arbitrary position in a file, you first need to figure out in which block(s) that data is located.
If the file is fragmented, this may require more reads than if it's contiguous and uses an extent.
Exact scheme is filesystem-specific.
Yeah, that was one of the reasons I said "just about". But minuscule compared to millisecond seek latencies on a spinning disk.
So the problem getting orders of magnitude smaller while the costs of the cure increasing a lot swung the equation away from defragging being a good thing overall.
The reason it was a problem on spinning disks, was that the delay in getting the next bit of data highly depended on where it was relative to the previous bit. With SSDs (as I understand it) looking up any bit of data is (just about?) independent of where it is located.
So, it's still a thing - but the cure (involving lots of wear) is a lot worse than the disease now.