Some say it doesn't change things. Some say it does. I had one female where she did change. She lost her power especially in the guarding. One female it didn't have any affect. Another was older and retired, but don't think she changed much and still had (heck, still does) the drive to work.
I like to wait until at least 18 months, but I don't find the heat cycles that big of a deal so tend to do them once they are older and retired.
I have worked my own spayed female who was done at 3 years old or so. No impact on drive at all and she worked until she was almost 10 I think.
I worked a spayed female that did not belong to me who was spayed at 1 year and we saw a massive decrease, another bitch who was not mine that was spayed at 2.5 and saw an increase in drive. She was actually spayed because the plan was to wash her out. It backfired, lol.
I think it may depend on many things. Age, lineage, and the individual dog.