I think it's different for different people. I'm a generally normal guy, not stupid but not extremely intelligent. I have a friend who is not one of the brightest in the bunch, and I've noticed his speech has become a bit impaired, and stutters happen quiet often.
He used to have impaired speech before pilling, I think that was from the weed he smoked even though he is a VERY light user. It all depends on how well developed your brain is and how tolerant you are to drugs, that's my opinion.
Personally, one or two days after dropping my sentences seem to scatter and I mumble and stutter a bit. But while dropping a hard pill it's very hard to focus on what you are trying to get across.
Haha one time, I was on a "practice date" through a shopping centre, and I wanted to try these new biccies I got, so I dropped the whole thing...And I was acting fine for like 10minutes after I peaked, but I slowly started stuttering, slurring and just stopping in the middle of every second sentence haha

. Got abit nervous i guess. Mind you, she was a 100% anti-drug person so yer...I told her in the end but she wasn't too pissed off.