Does it cost that much money to render frames in these animated shows?

I first notice it with Dragon Prince Season 1, cause that frame rate is really jarring, but then I started to notice it in many shows that is 3D animated. Ultraman goes from being a slideshow to being super smooth and back. It's really distracting, and I wonder if there is some kind of artistic thing I'm missing or if this is a way to save money.