Linux does not take 3 hours even on spinning rust. I'd say one hour at worst. On my computer with the compile happening on an NVMe, it takes about 15 minutes.
Because you're conflating kernel compile times with an entire OS? Linux userspace compilation can take 6+ hours very easily (try yourself with Gentoo).
At Microsoft, our massive servers churn out nightly Windows image overnight usually 5pm-10am next morning.
The Linux kernel takes 15 minutes. Compiling all of Linux (aka the equivalent of Gentoo emerge'ing e.g. gnome-desktop from whole cloth, which is what this is), would not take 15 minutes.
