@Harsh_Solanki it has to do with the anime S2.
If the series wasn't as popular as it is right now, kodansha would've told reiji to speed up the pace and tie the loose knots.
But there's development, and also a learning curve to boot (keep in mind both Kazuya and Chizuru are flawed characters and neither of them have experience with real datings or romantic feelings as far as their character backgrounds stared) . It's only that the weekly scheduling makes the story too slow and timing the manga events with the anime S2 debut is a hazardous task. Once you bingeread it, you'll realize there was a lot of progression, although very slow, but steady nevertheless.
About Ruka: Kazuya at first pitied her bc of her sob story about her bradycardia, but now he finds her annoying, self centered, pushy and demanding af. Ruka just loves her heart going dokidoki and uses kazuya as a pretense to make her heart go 90 bpm.
He has a hard time right now into making things clear with her bc one badly delivered word might make her heart collapse and kick the bucket.