This has been really informative. The many psychiatrists I've seen have never touched on this yet family and friends and strangers have all made numerous remarks about my speech (or lack of). Yes, my racing thoughts over the yrs are exactly like Robin Williams' racing mind (and my mouth could never catch up to my thoughts) and days and weeks of extremely poor mental energy makes speaking harder than lifting weights.
Personally, medication has never greatly contributed to speech problems and I've been on all of them over 30 years. It's always been the disorder(s) that have caused the problems. APs, ADs have caused many other intolerable side effects for me (akithesia, headaches, light-headedness, hot flashes, horrible moods, etc.).

I believe Robin Williams was definitely a manic poster child (much greater mania than depression) - no average person has those horrible extended racing thoughts.