It’s a mix!
Two of the three romances from Evertree are established in book 1 but deepen throughout the series. The third is a really slow burn, with the option to even internally decide your feelings not appearing until Book 2. For the three Sordwin romances, they’re all a little slower but I think consistent in their development (although Ruby is definitely the slowest of the three). Then Kyran/Kyra is only introduced in Lux. The pace for K is more like an Evertree RO though.
Hope that answers the question.