Generally, K-1 cases (more so than others) are processed in order of receipt. But as the late, great @geowrian used to say, "USCIS service centers are like a large grocery store. Some lines are just a little faster than others".
In all honesty, no case is the same. Individual background checks, individual adjudicator workload, etc. also play a part in the processing speed. The only instance I can recall of not generally processing in order was with I-751s a couple years ago. Patience really is a virtue when it comes to immigration matters. The journey is long and difficult for most couples.