There are two things at play with being too fast:
first, you know what you are talking about (thus are giving a talk) - and tend to want to give as much info as possible.
the second, and more important one, is the adrenaline rush that every performance gives.
It probably is this:
Consequently rehearsals help (they remove the worry about what to say)
Also one thing that helped me a lot is taking “dominant” poses five minutes before getting on stage. I can’t find the study that claimed it to work - but it did.
Later I have read the research that had claimed it didn’t work - https://www.ncbi.nlm.nih.gov/m/pubmed/28946020/ is one of the studies I could find quickly, but for me this technique still works, so maybe it’s the power of the placebo :)