Certain files take ridiculously long

Most prints finish in approximately the projected time that Preform predicts. Not exactly, but in the ballpark. But then every once in a while a print will take a wildly longer time. My last print predicted 11 hours, and it finished after 42 hours. As far as I could tell, it didn’t get “stuck” in any particular layer, they were just all slow. And I don’t see anything wrong with the print - all came out OK. This was with Preform 3.8.1. I’ve had a few files do this, and I don’t notice anything special about them that would make them different from files that print normally. They just print extra slowly.