To add to what others said - almost every major iOS / macOS release will be slower and drain battery more on the first day after install. The reason for that is they do a lot of indexing of your data and other preprocessing things that enable new features after you install. Once these processes end your device will go back to normal.

I’m surprised Apple does not communicate this fact more clearly to people, as many seem to be totally unaware of it (I do remember seeing notifications on macOS about that though)