I have experienced it first hand with both MacBooks and iPhones. That's enough to make me hesitate to click upgrade. It's certainly anecdotal but it's my anecdote that I would be a fool to not take under consideration. You should treat my story with more salt. Anecdotally a lot of people have experi need what I have and are also hesitant to update based on their own personal anecdotes. Take it as you will.
The other thing people need to remember is that our perceived experience is notoriously suggestible if you don't take efforts to correct for it. If you expect a new OS release to be slower, you're more likely to experience it as slower – and that first reboot will likely "confirm" it because every persistent cache has been invalidated, one-time upgrade tasks are running, all of the app updates which blocked on a major OS version requirement are installing in the background, etc. Very few people will take the time to measure before and after multiple times to know whether there's something objectively different.
It'd be educational if someone like Apple or Microsoft shipped an update which changed only the version number and then recorded feedback, particularly since there would be e.g. some percentage of people who had something like a recent hardware problem which they hadn't noticed and assumed was caused by the update.
