If you put on a bunch of muscle/weight, then you're going to be slower than you were when you were around around 30lbs lighter.
You think Usain Bolt would be as fast if he put on 30lbs? Nope. Desean Jackson? Nope. And neither should Roy Jones.
That's offset by the fact that Ruiz was a million times slower than anyone he ever faced. That would make him look even faster.
Jones began slipping well before this fight. He was such a great fighter that somehow managed to defend his title a couple more times, move up to HW, win a title, move back down and regain the title.
But there is no doubt that he was slipping. Flat-footed almost all of the time (a far cry from his prime), one punch at a time (he was once a combination machine) and then cutting muscle to go back to 175. All of that did him in.