Honestly, despite the older engine, Victoria 2 runs quite well with thousands of provinces and dozens of pops in each. The said, the pops are basically a spreadsheet of data and the game code makes changes (interactions, the thing that resembles trade, etc) happen based upon the data (plus a few other things that aren't directly attached to the pops). How well the game runs depends both on how complex the decisions are, and how many pops there are. I, for one, would be happy to sacrifice so many pops for better interactions between them, namely a decent trade system.
I wouldn't be surprised if some things are only calculated on one tick per day while the others are used to better scale battles. Shorter term event chains could also be calculated with the shorter ticks rather than the daily ticks. I'm no programmer, though, so I don't know if that would actually make it run more efficiently since it would then use two processes of ticking for events.