I would take the following steps:
Firstly, and perhaps most obviously, you may need to just buckle down and practice talking slowly. When you've got a whole lot of information in your brain and you feel the need to get it out, human speech can seem like a frustrating bottleneck. But you have to keep in mind that even if you talk fast, that doesn't mean people are able to process as fast as you can talk. And talking (in the workplace at least) serves no real purpose unless it's heard, and understood.
Secondly, Ask your coworkers for feedback. When they tell you you're talking too fast, stop and have a conversation, versus just letting them talk. Ask them what, specifically, would be helpful. They may simply say "talk slower" but if they're going as far as mimicking how you talk, they may be willing to think up some specific suggestions. Maybe they will suggest you put your thoughts in writing in an email, or something else. Whenever you get corrective feedback, it's always a good idea to ask for clarification if you feel the feedback is not easily actionable.
Thirdly, consider pausing before you speak and allowing yourself a second to collect your thoughts:
- What do I feel like I need to say?
- Why is this message important for my audience?
- How can I streamline my message to make it easy to understand?
I find this useful, personally, because I am sometimes accused of not just being a fast talker, but also of giving so much context and backstory that I lose my audience before I've even gotten to the point. Sometimes when I pause and think, I realize I can get my message across with a sentence instead of a paragraph. And, often, I've found that when people accuse someone of talking fast, they also mean that the person delivers too much info at once and it can't be processed. So, shortening what you say can be one way of addressing complaints of talking too fast.
Finally, consider interacting with your audience, instead of just talking to them. Watch their facial expressions and non-verbal cues. But, importantly, make your message interactive. If you feel like you need to blurt out several paragraphs about an issue with the Jones project, compare these two approaches:
Hey Boss, I was working on that deployment issue with the Jones project and while it was compiling I looked at our issue tracker for some input on the bug and I saw that the ... (and so on, for two paragraphs, without pausing)
Hey Boss, are you up to speed with the Jones Project?
(pause, and let your boss respond).
I wanted to talk about the deployment issue, because I'm having a problem with X.
(pause again, and wait for acknowledgement).
When I tried X, it failed like this. Have you seen that failure before?
The key to this approach is breaking up your message into short sound bytes and actively soliciting at least an acknowledgement that the listener is following you. Or, even better, ask them something and wait for their response. This way, you can have more of a back-and-forth versus just hitting someone with a giant wall of text.