Why does a portafilter machine take so long to warm up and a modern machine doesn't?
By portafilter I assume you mean the standard large coffeeshop espresso machine.

The Nespresso is only heating enough water for a single shot, which can be done pretty quickly. I'm pretty sure commercial machines take a long time for that first shot because they're preheating the large reservoir so it's possible to fully heat and pull shot after shot in fairly quick succession once you get going.

So the lag is because the bigger machine is heating a bunch more water--even if it doesn't superheat any more per shot than the Nespresso.

Somebody correct me if I have this all wrong, but that's what I was led to understand.