Comment by ack_complete

A major culprit is background processes that scan the drive in the background, like CompatTelRunner.exe. Works fine if you're on an SSD, but grinds an HDD to a halt. They also forgot about their own I/O prioritization API from Vista, so it also spammed I/O at Normal instead of Background priority in the early versions. Not to mention the periodic Defender scans, the Malware Removal Tool scan that runs before each major update, etc.

Similarly, Windows Update used to consume ungodly amounts of CPU time because the update system would write a multi-hundred megabyte text log and then spend forever compressing it for upload. Then they remembered their own ETL system and switched to much more efficient binary logs.

Firefox also has problems on HDD, I remember it locking up for minutes at a time doing cache maintenance until I switched permanently to SSDs.