> You may have ADHD

Don't think this is related to ADHD, I have very serious ADHD but I have no issue keeping up in conversations I am interested in.

ADHD is about where you can put your energy / attention, not if you are fast or slow when you focus on something. If you are very fast when you are interested then you aren't a slow thinker, if you struggle keeping up when you aren't interested due to lack of focus then the issue isn't being a slow thinker its the lack of focus.