Just one thing to add since this seems to be pretty new, are all of you having one on one dates with each other? Or is it always the 3 of you together? Relationships can develop differently and at different speeds. It can be hard to not compare the differences between your relationship with her and your husband's relationship with her. But since you are different people the relationship will be different. Enjoy what you have with her (even if it does end up being a metamour relationship) and allow it to blossom into whatever kind of relationship it is destined to be.
Everything will be ok in the end. If it's not ok it's not the end.