I have a lab quality Corning stir plate that I use for my starters with my 5l Erlenmeyer. I could never get the speed up over 8 (out of 10). It would religiously kick the stirbar off at a hair over that setting.
Today was my first try with a few small drops of defoamer in my starter. Now I can crank the speed all the way, and it runs like a charm. It is also noticeably quieter. It may be a co-incidence, but after 10 repeatable results, the only thing different here is the defoamer. Maybe something for people to try.