It was "efficient" by leaving almost no memory for user applications. I used two phones with 512 MBs of RAM each, one Nokia-something (620 or 625), and the other Asus-something (completely forgot the model, but it was on Android 4 and then 5).
WP would offload applications from RAM as soon as you switched into another application. It was impossible to multitask — you're writing a comment on a message board, switch into a dictionary to quickly look up a word, switch back... and the state is gone. If you're lucky and the application was written correctly, you would only have to wait for 5-10 seconds before you get your half written comment back. If not (which was the norm for the stuff I used), well...
The second Android phone had none of these problems, not remotely to the same degree.
It was such a widespread problem that it quickly became a meme on forums.
It seems like iOS is still fairly aggressive in killing background apps, a dozen years after the Nokia 625? I rarely feel like I can be sure that if I go off to look something up, that I can be confident that a half-written comment will still be there when I go back to it?
Huh, interesting, I've never had good luck maintaining drafts on mobile devices so very early on I got into the habit of drafting them in something like a mail or notes app. Sometimes I still slip up and start writing drafts in an app itself and then lose them if I get distracted for a minute, though it's more often because apps are too aggressive in refreshing their feeds (the LinkedIn app being a prime example).
The Lumia was such a great deal back in the day. An amazing camera for the time, a great UI, comfy to use and supported crashes as a champion. The last bits of classic Nokia legendary hardware. It's a shame that the Microsoft ecosystem was so limited in apps.
I would separate Nokia Lumia and Microsoft Lumia (the last batch). I was so happy with my Nokia Lumia that I eventually upgraded to a newer Microsoft Lumia phone. What a disappointment it was.
This also made the battery life much better. (Although whenever I mentioned this, the usual retort I got was, of course the battery life would be better if there were no apps to consume it...)