stallaris like all paradox game demands far more from the CPU to calculate
Fleet - tick for movement for every fleet on map, AI, player, monster per ingame day (you can see it when you use a really old CPU, hperlane will drop your fame by star to star movement on day base for all moments of the road....so every ingame day, as long the fleet moves, it send a tick to the CPU, wormhole use only 1 tick for system to system moevemet. I tested this on a athlon 2 core 3,2GHz....with warp or hyperlane, the game laggs from day 1....wormhole runs smoth until you start a fleet fight....only way to reduce handeling load is reducing speed)
Fleet - tick for combat, for every single ship in fleet, together with follow up tick if dogde or not, and the tick which animation to use per ingame day
POP - tick for ethics for every POP on map per ingame day (if they loose or gain traits)
POP - tick for POP growth
Leader - tick for dying per ingame day
Leader - tick for XP gain per ingame day
Leader - tick for level up chance on trait
etc...
so smaller map, less action happens at the same time, far better CPU handeling, far better speed.
many players with high end systems run simply in lategame into the CPU handeling limits of there CPU...