Yeah the -ac kernels were for quite some time what the hep cats were all running. Alan Cox did a ton of different performance improvements that slowly made their way into the main kernel over time. I also remember they were way better if you had large amounts of memory for the time.

I also remember this weird little side note when two different teams were both working on some sort of device management subsystem, and when the kernel team selected one and not the other, someone wrote this really touchingly kind note to the other team. Like look, your system is perfectly good, it's easily deserving of getting merged and it's gonna suck that you worked hard on it and it's more or less getting thrown away, but we have to pick and standardize on only one system. But please understand that it's perfectly good and we're not saying it as any kind of value judgement and we hope this doesn't discourage you from contributing good work in the future. It was again that same kind of lesson as with Reiser or BitKeeper that you have to keep the human element in mind.