Not research, personal experience:

Even after many years of school/high-school in basque, I learnt it at a way slower rate than English, which was just 1 subject.

I didn’t speak neither basque nor English outside school. At most, the difference might be that I consumed a little bit of media in English while none in basque. But all subjects except spanish and English were in basque, so that should make up for the difference.

And I don’t think it’s just a me thing. Since the curriculum has mostly been the same for all those years of school:

Learn how to say a verb.

That’s it. Many years of school just to say verbs correctly.

The exams where mostly just fill in the blank exercises, where the blank was a verb.

I still don’t know how to say verbs that aren’t the simplest ones.

So to your question I’d say yes. Even though neither are my native tongue, I learnt both since I entered school, but learned them at wildly different rates.