It takes him years to get through a single sentence when he's on the stand, and its a bit annoying how he tries to use such fancy wording to describe something, he talks like some exotic artist describing his philosophical view on a painting or something.
I've seen him on TV shows in front of live audiences, and he was fairly fluid in the way he delivered his responses. Admittedly, this was around the time he first met Heard. I guess he elevated his level of drinking as a response to the marriage. His brain is certainly damaged due to all that alcohol, and slower than is once was.
It seems to be in him to often pause shortly while he's talking, though. Like his thoughts sometimes stumble onto an obstacle before they get out. Now, like you say.. the pauses may have gotten longer with the years. And maybe the stress of talking at a trial didn't help either.. especially when your own image and reputation are on the line.
Even young Johnny didn't really have a natural fluid speech. (I've used this video as an example in another thread as I think it shows that's just how he talks.)