A couple things to say about this, a lot of people will just use vents to bring it up to temperature. Either way works. But when using the Guru you need to have all other intakes sealed. Otherwise the forced air will find the easiest exit, which will not be past the coals and up to the top vents, it will instead simply flow out the other open intakes.
One thing to note is the position on the fan damper. Assuming you have the 10 cfm fan the damper should be about half open I would say. Your damper may be choked down too much making it heat up slower than normal.
XL Big Green Egg, Weber Performer Platinum, WSM 22.5"