For me it is more of a psycological thing. My therapist told me once that everytime we go back to our old lives/parents‘ house we return to our childhood behaviours and habits. So I learned to speak Portuguese as a child and back then I wasn‘t able to control my stutter and within time I have learned English and German and even though my vocabulary in both languages are smaller, which makes it harder to find synonyms, I still find myself speaking more fluently than I when I speak Portuguese with my parents, because most of the memories I have attached to my parents are when I was a no control stuttering child. Does that make some sense to you? Also I can‘t say there is a huge fluency difference between the languages, me talking to my parents and me talking to my friends that strangers would notice, but of course that as a stutterer I notice every small change. That was at least the explanation my therapist gave me after I spent a year doing intensive speech therapy and by the end of the program I was stutter free but a few months back home with my parents it had all came back.