There you go, mystery solved.
In Audacity versions above V2.1.0 there is a problem with "Change Speed".
Values you might enter for the amount of speed change get changed, without any notice to you.....the value's digits get truncated for some damn reason.
Say, if you enter 0.001500652783961 for the factor, the program truncates the value to 0.0015. WRONG.
Go back to V2.1.0 and I predict you will be pleased. Why the team broke something that worked properly is anybody's guess.
I never asked anyone why, I'm just standing firm on V2.1.0 until I hear this regression has been addressed.