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.
He wasn't gun shy on the Pulev fight and he won via KO if I'm not mistaken.
But in the Usyk fight, it seems that we witnessed a different version of Joshua here, maybe it was the style match-up, maybe he has ring rust, overtrain or underestimate Usyk etc, etc.. He usually dominated small fighters except in the Ruiz fight. Even in the second fight, he adjusted and regain back his confidence. So I doubt that he is still affected by the first lost against Ruiz.