Optimising your Mac

You don't; that's a normal process and will go away once it finishes. Just keep using your Mac while it does its thing.

The admonition about performance and battery life is a normal disclaimer. The Mac is working in background and like every other process can affect performance and battery life to some extent. It's usually very slight for both.

Hi, I've got the same problem, after using migration assistant to transfer the contents of my 2013 Macbook Pro (Catalina OS - this model can't be upgraded futher) to a newer, clean 2019 Macbook Pro (Ventura OS). I updated the software on both before backing up on an HDD Time Machine and then doing the migration from the Time Machine. The migration (about 350GB) took a few hours, but when I signed in to the new Macbook, I got the "Optimising your Mac - perfomance and battery life may be affected until completed" message. This has continued for 24 hours so far. You say above "Just keep using your Mac while it does its thing." but I can't, as the screen only shows my User name and a circling wheel - there is no means of accessing the contents of the Macbook with the mouse - are there any key combinations which would allow me to do so?