I would give it some time.
Though I am newer around here, I am a roadie with some experience. Same thing over there . . . your body will fight you in a new position doing new things. I do not care who fits someone to a MTB, even with every custom part, the new rider is going to have A) hands that hurt and B) neck or back pain. They are also going to be uneasy on single track trails. They front end might be squirrly. They may go down a few times.
IMHO making changes now is not the idea thing. It might cost you money and it will only add to the doubt that some part is the reason you are not riding faster through the trails. Give it a handful of tries and then start to look at adjustments.
Just my $.02. After 6 or 7 rides this season so far (just got my MTB this winter) it feels like night and day since the first ride.