I go with Steve with the freehub.
If the chain and cassette are new, it ony leaves the chainrings and freehub.
As an example, FWIW, my Raleigh chainrings have done 3500 miles plus and are showing wear...but dont slip under load.
Yet...the freehub on my Bianchi was slipping a bit a while ago...not that much mileage done...but then again, it hasnt done it for ages.