@hongminhee I would disagree with the conclusion. My experience is different. I was perfectly bilingual as a kid. Korean was, in a sense, my first language together with German. But my Korean is not that good today. Now I am a language learner, mainly BC my vocabulary is too small.
Now to my point -> Though I experience that same exhaustion and slowness of thought, Korean triggers a lot of emotions. On the other hand I can talk English all day but I think I feel its 'utilitarian' character as a second language.
I think it depends on how much you can connect culture and memories, and also if you grasp all the concepts a word can carry, not just the literal translation.