I highly suspect bitters (and their pathfinding) to be the cause of the problem so i tried game.forces.enemy.kill_all_units() and woah, the UPS was back at a decent level (~45).
As I want to keep achivements unlocked, i didn't played after killing the bitters to see how long the ups stays ok (So the save is basically free of any commands).
Anyway, i'll post the save if you're interested in profiling and optimising stuff and because there might be something wrong going on as I once experienced : viewtopic.php?f=11&t=15238
Normal gameplay
after kill_all_units + 10s
factorio-current