Thread scheduling has existed since NT 3.51.
Wifi is largely just another kind of network adapter.
XP supported the same suspend-to-RAM and hibernate-to-disk modes as todays systems.
"but I haven't forgotten how poorly my old XP two-thread laptop multitasked"
Despite what certain processor vendors tell you, the number of threads are determined by each running program. Whatever you mean by "poor multitasking", you were probably short on RAM.
The OS has made improvements that can help with battery life, but most of any perceived improvement has come from batteries that suck less.
"In the XP days, you were running Firefox 1.0, a 5MB program."
5 MB was never a realistic estimate of process RAM.
