I'd say so. Mistlands was originally going to be different from what it is now, so they went through redesigning the biome which most likely contributed to the time it took to make. Not to mention that it was a step up regarding things like enemy, dungeon and weapon design, as well as new mechanics like Eitr and the mist. They're definitely more experienced, and iirc, they were originally non-programmers, and there's a steep learning curve regarding that stuff. And also, they added many smaller content updates before mistlands, so they weren't 100% focused on the biome at the time. Looking at them now though, they have their eyes set on Ashlands, and maybe some small additions to the other biomes if we're lucky along the way.

Though i'm not too certain but there may also be an Ocean biome update ? Though i think it's ashlands, then deep north in queue first.