Awhile I suggested that part of his slump may be due to change of rackets. I then was berated here for believing in some marketing hype. FU.
When someone shanks that many balls, there is something off with their timing. Feds timing is usually spot on. I attribute it to that there was a genuine change in rackets. It is one of the only variables that has truly undergone change.
If anybody does a search, you will find that Wilson and Fed went through many molds to get "it right". I personally don't believe it is the previous racket with a new pj.