I had to do this when acting as a customer service phone rep at a former job (yeah, I know). I found it to be incredibly exhausting. I wasn't necessarily just slowing down, I also had to focus intensely on not allowing my larnyx/throat to lock and give me a block, and speaking at a normal pace, and not repeating, and avoiding words that would cause me to stutter while finding substitutions. Plus the actual customer service end. I'd get off a call and put my head in my hands for maybe 30 seconds, deep breathing and trying to recover. There's a subconscious thing sometimes where you're basically trying to get the words out before your speech production facilities start fighting you. I definitely know how that feels. But that also make you tense and set the stage for stuttering. Not a criticism, just one of those things that happens. A professional might be worth a try, even if it's to understand how stuttering works in your particular case. You may find some added fluency, or might just understand it more which does make you feel a bit more confident.