I've been diagnosed in the past with ADHD inattentive type, which can include the feature of sluggish cognitive tempo (SCT), and it was certainly a predominant feature for me. SCT is symptom cluster that's also looked at as possibly being a distinct disorder, but the research on that has been pretty limited thus far.

My most recent major depressive episode has resulted in distinctly slower cognitive tempo for me, and for a long time I tried to compensate for it, tried to power through it to achieve the kinds of levels people were used to seeing from me, but in doing so I just caused myself energy debts I couldn't repay. I've definitely re-calibrated my life around it, accepted (at least for the present moment) the change it represents. Not happily so, but at some point I just decided I had to go with that adage of "start from where you are", because where else am I going to start from, really?

If you need something to tell people when they say you're quiet (which would annoy me, personally), you could just tell them something like you suppose you've become more thoughtful.. accurate enough but not personal. I'm actually pretty social, but even at a party I'm usually only chatting with 2 or 3 people at a time. Fighting to get a word in edgewise in a complicated group situation is not my style...

Good luck