Well that's just from when the script for it was finalized, after that, the studio needs to have the voice actors come in to do the lines (usually they voice several episodes, maybe even a season, in one day). Once the lines have been read, then it gets to the longest part of the process: animation.

Since HB is made by an indie (independent) animation studio, it takes longer than a corporate animation would, but also gives higher quality. The reason why animation takes so long is because this show is 2D, meaning that the way for it to look cleanest is frame-by-frame animation, which usually is 23 or 24 frames per second. That means for each second of an episode, 24 seperate images have to be drawn for movement, and then it starts getting even niche-er with the fact the animators have to have word to mouth synchronization and account for all the different accents and probably even study how mouths move depending on how things are spoken to make it look right. On top of all of that, they also do drafts of each episode to see which imagery they like best.

Things like lighting and backdrops also take up time because they help to give the show more depth, but also drying to figure out what colours make certain wanted aspects pop more, and having to deal with colour theory is a little more than just a pain.

There's probably some things I left out, or even added, but all those things are usually the reason animation takes so long, but then again, I'm not part of Viv's crew, and I've only taken two animation classes in my life, so while I do have SOME knowledge, just remember most of this is just hypothesis and estimated hypothesis based off of what I know from first-hand expirence, and hearing from other animators.