Hello, NikMarkVT
Welcome to Microsoft Community.
I realize that you have encountered a problem with PowerPoint. Although you have used the “Rehearse Timings” feature , the Transition will delay after inserting animations to the slide. I understand how you feel.
We've tried to replicate the issue here, and we encountered the same issue when exporting the PowerPoint file to a video. This seems to be design-related issue. PowerPoint seems to incorrectly associate the mouse click time.
Unfortunately, there isn't a temporary solution for this type of issue in the short term.
I sincerely suggest you make your suggestions by filing a report through PowerPoint · Community (microsoft.com).
Click Send feedback and share more about your issue there.
Additionally, directly set the time for transition to the next slide in the option in the image below.
In our tests, set in this way and then the "Slide Show" seems to show no issues. If this works, you may record the screen with other software and take this as a workaround.
I’m fully aware of the inconvenience this has caused. I hope that the PowerPoint development team can fix this feature in the future.
Thanks for your understanding and patience.
Best wishes,
Lucia Wu - MSFT | Microsoft Community Support Specialist