I just did my first butt last week, coming in at 7.9 lbs. Main thing is cooking temp. If you dont know than invest in some thermometers cause otherwise your stuck guessing. Though no matter how you do it, its done when its done. I did mine at 2:30 am, as i recall sometime around the 8 o'clock hour it was around 150 i believe, it than made its way to 160, froze there for a few hours...crept up to 170 and froze some more. I pulled it at 5pm at 193F (14+hrs!), i wanted 195-200 but we needed to eat. Temps stayed around 220-250 the whole time. So no i dont think its going too fast, the hardest part is getting it from 160 to 190 because this is where the fat actually breaks down and until it does it will maintain temperature.
This is assuming indirect heat, otherwise disregard everything ha!
Good Luck and I'm sure it will turn out well!!