It's not that AJ is slower than Usyk, it's just he cannot fight a southpaw boxer the same way as he fights an orthodox boxer. The tempo, the feel, the angle, everything is so messed up. He cannot get used to fighting a southpaw boxer in just a few months, so better gamble would be going for in-fight against Usyk.
Maybe not slower, but definitely less agile. I think that's the biggest issue here, and the fact that Joshua seems a little more gun shy since the Andy Ruiz fight. Until Joshua can either counter those problems, or make improvements on those fronts then I can't see him winning the rematch, and it's looking increasingly likely that it'll be a Fury vs Usyk unification fight, which to be honest I think is a good match up. Although, I'd like to see Fury win that, and then move onto Joshua just to prove he's the best of the current heavyweight fighters.