Nadal is still very fast, easily one of the fastest on tour today. However if you doubt that his speed has declined a large amount from his younger days in say 2005-2013, especialy 2005-2008 before his first serious leg injuries, you did not watch him play enough then. His speed was out of this world back then. The other thing that has declined is his unbelievable ability to turn defense into offense. Yes some areas of his game have improved. I don't know if I would say his forehand improved, I think you are underrating how formidable his forehand used to be, and overrating his current one some, but those others things you mentioned largely yes. However what was most effective in his match ups with Federer, Djokovic, and I would suspect although it is just conjecture mainly Agassi, is his insane speed and ability to turn defense to offense. It shows in his inability to win a single match off clay vs Djokovic or Federer for so long, although he used to beat both off clay often years past, and even though Federer is much older than he is. What else can you attribute that too.