Share 'Player joining mid round affect your game?'

Today, at Barfield Crescent, I birdied the first three holes. I player joined the game and my game changed drastically. I proceeded to bogey then double bogey. I manage a few hard pars then he left after hole 9. My game miraculously recovered the rest of the round. I have heard other players say that they will not let a player join mid round. I presumed it was for a reason similar to this. Any th…