They also review the case to check USCIS included everything and there were no errors in their adjudication. I imagine they also need to enter things into their system and then liase with embassys. It is entirely the embassy who decides if and when a case comes to them. NVC can't ship without the go-ahead from embassy. They are also handeling numerous other case types which are more intensive. K1 is a drop in the bucket to them, they have more going on that looking after your case. You're in a queue, it takes the time it takes.
Average is 3-6 weeks with 5-6 weeks being most common.