
Why is she so quick with naska but so oblivious about arth?? It's so weird

Firstly, it's because Naska straight out told her that he's in love (his heart is pounding). All she had to figure out was WHO it was he loved, and given the options are only three people it's pretty easy to deduce.
Furthermore, Naska isn't a noble. It's clear that Sasha doesn't have much faith in the sincerity of the affection she receives from her noble-born associates...rightly so, for the most part. Even Arth and Rebecca; although she knows that they do genuinely care about her, she still holds herself part of herself back from them.
I think it's hard for Sasha to believe that she can be truly loved. Because Naska isn't human and hasn't seen much of the world/interacted with other people, it would be easier for her to accept that he developed feelings for the person who showed him new things, as oppose to accepting the idea that someone like Arth who could have much better prospects than her would fall in love with her.
My soul has been cleansed