hi Harri P,
First the basic question.
Did you change anything in Mainstage or Mac configuration? Is your Mac only for audio or regular use?
Second question 🙂
When you say delay before ultrabeat starts up, you mean when you start you Mainstage Concert the ultrabeat as a starts up delay, or when trigger the pattern on a keyboard?
When play some notes on other patch (like piano) do you get that delay?
You could copy/create a new ultrabeat channel strip to a new concert and see if you still get that delay.
You can send me your concert to see if something´s wrong.
Post your results.