That was my experience as well. My understanding is that the simulation will be bottlenecked by the slowest PC. All of my friends had pretty powerful PCs, but someone among us had a slightly weaker one in terms of single core performance, which means we all have that CPU basically then.
The huge battle toward the end of the game dropping or making the game unplayable was what killed it for us. We didn't want to spend all that time making our army and strategy only to be disappointed. At the time we went back to playing Age of Empires 2 DE.
> My understanding is that the simulation will be bottlenecked by the slowest PC.
That's correct. While 0ad is an RTS, behind the scenes it's still turn-based and in multiplayer games turns can only progress once every participants PC has responded. That's also why the game also feels slow when a player has a bad latency to the player hosting the game.