Shogun wasn't exactly speedy at that stage.
True, but he's looked like a completely different fighter since that fight and the only thing that happened between the Shogun and Machida fight is:
A) A year
B) Knee injury
Against Rashad he looked a lot more nimble than he did versus Machida (still not quite as agile as he was against Shogun though). Now even more time had passed, but no injury, yet he got faster. Therefore we can rule out time and age being the reason behind his slowing down between the Shogun and Machida fights as he got faster with more time in this situation.
So now that we've ruled out time as the factor that's noticeably slowed down Henderson lately, it's therefore not exactly a stretch to assume that it's his injured knee that has held him back as that's the only other difference.
Against Shogun:
Against Machida:
What do we see in these images? Against Machida, after having injured his knee, Henderson wore a knee brace. Against Shogun, prior to his injury, he didn't wear a knee brace. Against Shogun, he was quick. Against Machida, he plodded. It's therefore logical to assume that his knee wasn't 100% or back to what it was prior to the injury as it needed a brace.
Now I've already said earlier that Henderson was faster in the Rashad fight (although not as fast as before) and here is a picture of him from the Rashad fight:
Note the lack of a knee brace. His knee was much better and it showed in his movements. Still not a 100% (maybe won't ever be), but we can't play his slowness on time as he got faster. We can't even say he was faster as a result of his TRT as against Rashad he wasn't on it.
Ergo, I expect to see a much faster Henderson than was displayed against Rashad and Machida as his knee has had more time time to heal. I still expect him to lose though as Belfort's in general way faster than him, but I don't expect it to be because he's plodding around as he was before. I just see Belfort as having too much power and being far too quick with his hands (and kicks now apparently)