I'm referring to the big punch he landed on Travis toward the end being the one that shouldn't have been hard to avoid, not every punch in general. Yes, his striking is slow, but it's not bad.
Brown backpaddled after an uppercut by Silva and bumped into the fence. In that situation he wasn't really free to move, regadless of his knee. So it would have been hard for anyone to react and avoid that punch. And while his punches look a little slow, he covers quite some distance with them. Those guys are no flyweights, they have a lot of mass to accelerate when they want to get out of the way.
What does look weird in my eyes are Silva's GnP punches with his extended arms without bending his elbows. It looks somewhat uncontrolled to me.