Which screen set are you using.?

In the macro that runs the Auto Z code there will be code to change the speed.
To find the code and change go to Operator Menu and select "Edit button script". Then you should see some of the buttons on the screen flashing. One of these will be the button you click on to AutoZ. Click on it and it and inside the script editor it will show you the code for the AutoZ. Depending how the code is written then there will be a feed rate value F500 for instance.!! Change this value to a lower value.
From what you say it sounds like your code just use's a single speed which is a bit basic and not very accurate.!! Better way is for the probe to do 2 checks, first a fast initial probe to get close then a slower one to be accurate. The second probe retracts slightly then does a really slow accurate probe then sets the Z Dro to zero then finish's by going to a specified retract distance then.

If your not sure what I mean or don't fancy messing with code then If you zip and post your Mach profile by going into the Mach folder and finding the XML file with the same name has the profile you use then I'll take a look at it for.