It's interesting that you say that, because one reason I never tried 4e is I realized converting classic modules to 4e would be more work rather than less work than converting them to 3e or 5e.
But even more so than that, the vast majority of prep that I do in any system isn't mechanical preparation. I think I should probably fork out to a thread explaining what takes so much time to prep, but it isn't doing numbers for monsters or NPCs or whatever. That's the easiest part of prep. The map making tends to take more time than stat blocks. But the hard part of prep is the writing.
So whenever someone tells me that they breeze through prep in 1/10 the time by changing systems, it mostly just informs me that we have very different ideas about what play is like and what part of it is hard.
That said, I have no idea why people think prep in 4e is easy. I've tried my hand at it. There is a thread around here where I give advice for creating Tharzidun in 4e and even the rough, unfinished, and somewhat unsatisfactory draft of a combat with Thardizun for 4e easily proved itself the equal in intellectual challenge to creating that entity in another edition.