They acknowledge it is not optimized properly yet.
I understand it is hard to keep it fully optimized, as it is so quickly evolving.
Having said that, the mod does have quite a few more features, has many more operational controls, and has a much more realistic flight control response, all which flies and handles noticeably nicer than the default. All that is going to cost some fps.
I think I read that they do plan to do a good optimization once they reach a particular point, and I think they are quite close to that point, if remember correctly.
You should be able to find more info through the A320NX mod’s home page, and in the “Issues” and “Discussions” sections there.
Here is a link to their site.