DS1 (7) does this.
He says "Mummy can I ask you something"
Me: Yes
Him: Sooo... umm, well.. I need to ask you about... umm
Me: Yes?
Him: I need to tell you... ummm can I please, you know...
Me: What?
Him: You know, can I please ummm...
ARGH!! He has no stammer. He has no LDs he just lacks focus and concentration.
I can't use email. I tend to shout at him if I'm in a hurry and he's dithering around - which does focus him back on point, but he's my son, not a colleague. I wouldn't do it to a colleague! Probably shouldn't do it to him either but it's very frustrating.
Australia - ha, I don't find people talk more slowly here, but they do feel the need to repeat themselves at least 3 times when they tell you something. So you hear the story once, and respond appropriately and then you get it again, and you're thinking "yes I know, you just said that" and then the 3rd time it's getting a bit strained! One friend, it's 4 times. Every Single Story - 4 times in the telling. Not at different times, OH no, all in the one sitting. In case you missed any of it the 1st, 2nd or 3rd time around. She is lovely but oh God it's frustrating!
Aussie Dh does it too - less so with me now unless he's been away on conference or something, when he reverts. I'm even, God forbid, finding that I do it myself, although I never go beyond saying it twice!
For you, OP, you need to find a way to handle your colleague better. You can't really ask them if there's anything they haven't disclosed, but perhaps passing post-it notes if you need something in a hurry, or if you have to take a phonecall would help. Prior emailing would certainly improve their thought processes, because they'd have advance notice of your intentions and be able to plan a response. They might have anxiety over making decisions, so that would help that too.
In the end, you are the only one who can control (or not) your frustration with your fellow - yes, it would frustrate me too, btw - so finding a way to manage your frustration is the only thing you can realistically do.