I don't think it's necessarily grass. If it were a choice between the 4 slams, then yes, Wimbledon would be the one he's been underperforming at the most. But if there were more tournies on grass, I don't think he would overall perform the worst there. For example, I think Fed has better chances against Nadal on grass, than on slow HC, for example.
I think it's close between grass and slow HC.
The Fedality: 30 Slams, 8 YE #1, Olympic Gold in S&D, 6 YEC, 47 Masters, 140 Titles
RF: 17 (4-1-7-5). RN: 13 (1-8-2-2).