For what it's worth, this change happened because people were seeing the stat() call involved in startup profiles, taking sufficient time that it seemed worthwhile to avoid it if possible, as far as I can tell.
C'mon the directory structure would be cached already, even if it was not - the seek times for HDD are 3-4ms.
If your HDD has a seek time of 3-4ms on average, that means it needs to rotate completely in at most 6-8ms, which gives you a rotation speed of 7500-10,000 rpm. HDDs in data centers do that, sure. Consumer HDDs just don't do that, last I checked; they're mostly in the 5400-7200 rpm range, with laptops firmly in the 5400 bucket. See https://www.dell.com/en-us/shop/dell-laptops/sc/laptops?appl... for example (currently offered Dell laptops "for home" with an HDD: they're all 5400rpm). At 5400 rpm, your average latency from just the rotation is 5.5ms and your worst-case latency from the rotation is 11ms. That doesn't include other latency sources, but let's assume those are somehow scheduled away to happen during the rotation.
Keep in mind that what typically sticks in users' minds is worst-case, not average-case, behavior, so you have to bring your worst-case time budget down to whatever your target is.
Yes, there are. They are not a niche cohort.
EDIT: In fact, we have many more users with magnetic HDDs than who use userChrome.css. That tells you everything you need to know about this decision right there.
