It generates a new address and puts it in the main window for you if a payment is received for the previous address. This is so each time you click ‘copy to clipboard’ to send it to someone, it’s easier to get a unique one. The old one is still valid and it’s still in your address book.
Try renicing bitcoin and see if it helps. Find the pid in top or ps and then renice +19 1234 where 1234 is the pid.