APFS is designed for SSDs and flash storage. It avoids fragmentation aggressively, but if you have an external HDD formatted as Mac OS Extended (HFS+) , fragmentation can still happen.
When a file is (not fragmented), the head reads it in one smooth, sequential motion. When a file is fragmented , the head must: defrag hdd
Defragment / optimize your data drives in Windows - Microsoft Support APFS is designed for SSDs and flash storage
The read/write head has to physically dart around the spinning platter to piece the file back together every time you want to open it. This takes time, causing the "clicking" sounds old computers make and resulting in system lag. When a file is fragmented , the head
A well-defragged HDD is a happy HDD. It won’t be as fast as an SSD, but it will be as fast as that HDD can possibly be .