It didn't have cgroups, because at the time it was thought of as a distributed system meaning it would run a single service, authentication for example, and the

It didn't have cgroups, because at the time it was thought of as a distributed system meaning it would run a single service, authentication for example, and there was no real ability to share resources in the way we do today. Remember CPUs were largely single threaded, and you had expensive multi-cpu systems that ran 2, 4, and 8 CPUs. Compute heavy processes consumed entire systems, or CPU since SMT was also not common. You couldn't effectively microsegment CPU usage, because you were only really multiplexing idle time. Additionally, your OS had to consume more cycles to do that resource sharing. Today, the story is pretty similar, but we have systems with multiple cores, and SMT. We can share resources more effectively, but its also that our systems have 16x more scheduling slots.