Messi's never been lightening fast, he's deceptively quick, great burst of acceleration, loads of body feints and quick feet.
He still has that, just his general movement is poor, he seems to refuse to chase players down and create space for himself, ergo people say he doesn't look the same and start exaggerating how quick he used to be and how much running he used to do. It's bullshit.
Next season he'll be back to scoring 50+ goals, Barcelona will look a rejuvenated side and the same people saying he looks like he's on the decline will be blowing steam up his arse again. I wouldn't worry too much.