go use it for a week or two first to let everything settle in.
on the push to go it adjusts the pulley size which effects the tension on the belt
as the belt wears it loses tension and the mower loses speed
just like new shoes sometimes belts need a little break in time as well.
i think you are fine but if its still doing it after a couple of weeks of cutting, then check back in with us because you might have a problem