IMO - no single mechanical factor alone can account for a very noticeable difference in acceleration, except maybe tire pressure. It would be a stretch to imaging multiple factors to exist simultaneously without being observed.

I have to wonder if there's a difference in gearing. If for some reason, you're riding a higher gear on the newer bike, that could have a very measuring effect on acceleration. For example if the older bike had a 53/12 high gear, and the new one a 53/11 and you are riding comparable gear positions, then the higher resulting gear will be higher and harder to accelerate from a standing start.