Hatton and Oscar are much better opponents than Marquez and Mosley, taking into account the respective conditions of the fighters at the time they fought him. But yes, I thought he looked faster then. I can't really say much to persuade you if you disagree. On the contrary, I think old man Mosley and pot-bellied JMM flattered Floyd's speed. He is/was so fast that it's difficult to notice that he's slowed down, rather like Roy Jones, who slowed noticeably into his career at 175, IMO. I think you're underestimating the extent to which Floyd relies on his legs for his defence. From my observations, most of the time - from Corrales to DLH - Mayweather generally fights by keeping a safe distance from his opponents and out-timing them from that distance rather than pure counterpunching. When he's put under pressure (vs. Hatton, Castillo), that can change, and he spends more time slipping, blocking and getting into serious exchanges. Now, Martinez is somebody who can match him for speed of foot and hand, and he's going to be forced to get into a timing match with somebody not only so, but who is pretty unorthdox, southpaw, and possesses physical advantages over Mayweather. Plus, he's bigger and stronger. It's difficult to counter somebody like that when they're just pouring fire over you. I'm not claiming Martinez would shut him out, but I think he'd do enough to get the D. I think Pacquiao could beat Mayweather for similar reasons.
I think it's safe to say that only haters and trolls would refuse to credit Mayweather in a big way were he to upset Martinez. The better question, in my view: how much credit would Martinez net for beating Floyd in a dominant manner? Would it be about the same as Hopkins earned when he was middleweight champ defending against a still-close-to-prime DLH moving up? More? Less?
I agree with the idea that Floyd's declined just ever so slightly physically. He seems sharp, though, and he seems very high in confidence and his timing. But his legs aren't the same as when he fought DLH. He just was much lighter on his feet I think. And his hand-speed while still great, is not as great. Still Floyd might be smarter and mentally better than ever. This plays a great role to him. I think that fighting Martinez would be a huge challenge and I would give him more credit for that win than I would for his win of Mosley and Marquez. I'd still rather he fight Pacquaio.