This is to do with large ships docking, many ships at the same time entering a station boundry box and active turrets at a large player base station.
I have spoken with Lancake about this prior to reporting it here.

Example 1)

Player has 500m ship with 15+ turrets (deactivated) and goes to dock at their station. When the ship nears the station and enters the stations boundry box the client fps drop very low maybe 3-5 fps until the ship has docked then it returns to normal.

Example 2)

NPC pirates spawn (15+) near a player station and the station turrets are active fps drops, when the pirate NPC's enter the stations boundry box the server PING goes crazy 1000+ and is reported by other players and the clients fps dropped to 1. It was necessary to admin mass depawn MOBS to stop the server locking up.

Big vs Big is actually not really an issue. However, it does become one if you have docked entities and all of them come "close" to that other big contraption.
This could be a station you're just approaching with your big ship + its 50 turrets. Once enough of those turrets come close enough you'll see a giant impact on performance.
Or 2 big ships face hugging each other with plenty of turrets near the front.

Looks like an issue where it just scales badly, the more docked entities you have, the bigger the performance drop when you come close each other. Not entirely sure if that is caused by a bug or if it is just a physics limitation for now.