I have another bug who slipped through the net to report.
The game starts lagging when I talk to the digger slim in the casino after I fighted it when it was blocking the path.
I had saved right after the fight, before I talked to it and the game started lagging, and when the game started lagging, I saved on another slot then shutdowned the game, relaunched it, and reloaded my 2nd save.
The game was still lagging.
Then I reloaded my 1st save.
The game doesn't lag anymore.
Conclusion: Talking to the digger slim generates a bug which causes the game to lag and keeps lagging if the game is saved, shutdowned and reloaded, which means it is a persistent bug.
So may I suggest you to check the script of the interaction with the digger slim in order to find and fix what causes this interaction to generate a persistent bug?