The directory structure may or may not be cached already: these files are in a separate directory from everything else Firefox needs at startup. If your HDD ha

The directory structure may or may not be cached already: these files are in a separate directory from everything else Firefox needs at startup. 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?appliedRefinements=13242 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.