I'm convinced a lot of that is because Kira goes from "hi, we've barely met" to "we've spent weeks at sea together" without much player time in-between, but Luna and Alice have a lot of interludes with other characters and plot between their scenes. So even if their total time is similar or less, it's spread out more.
There's also the issue that Kira's tsundere schtick requires her to reverse her initial position. Alice was pining after the MC for years, and Luna started neutral and warmed to him quickly, so it's no surprise they get all touchy-feely very quickly; it's a straightforward continuation of their initial arc. But Kira makes a huge show of how NOT into the MC she is, yet she proceeds to get even more touchy-feely in a very short amount of screen time.
Sure, we know her initial act was protesting a little TOO loudly, but she still has to overcome that internal block before the relationship can get off the ground. That should slow Kira down at least a little bit, but in practice it doesn't. Combine that with the story skipping over the weeks they spent traveling together and it's easy to see why Kira's path feels so much more rushed than some of the others.
