I feel like there are 2 parts to processing: pre-snap and post-snap. Pre-snap can likely be fixed with good coaching and repetition. Post-snap is harder to fix. But if he can be able to read a defense prior to the snap, the post-snap processing will maybe limit his ceiling a bit but his physical gifts will still allow him to be a very good QB IMO. Josh Allen was criticized quite a bit his first 2 years for having slow processing time but obviously managed to become an elite QB. Cam Newton also struggled to go through progressions for much of his career but his speed and size allowed him to overcome a lot of that and become an effective QB.
It's not a great sign if Caleb is struggling to go through progressions, late on reads, etc but there's a reason he was the 1st overall pick, he has elite traits that could allow him to overcome this if it continues to be an issue.
I'm not really reading much into anything preseason anyways, he's basically going through boot camp trying other erase everything he learned as a rookie and trying to run what is a pretty complex offense. Unlike Nagy and Eberflus, it doesn't seem like they are trying to fit a square peg into a round hole, Ben Johnson has stated repeatedly that the offense is being build around Caleb's strengths and based on his track record I trust that he will follow through on that.