As slow as basically every hard court on tour these days
As for what benefits Federer, obviously fast courts. Even in his prime, defense wasn't his main asset (although he was great at it), he always beat big hitters by taking the initiative off them, taking their time away (especially those who take huge cuts at the ball ie Del Potro). Against the best defenders (Murray, Nole), it goes without saying that a fast court would be a huge benefit. The only player on the draw against whom fast courts could do more harm than good for Federer would be Tsonga.