sweep speed?

Take into consideration that not every signal in the ground is the same as waving a clear target in front of your coil. What I mean by this is that in the ground a good target ie; silver, might be next to or underneath a bad target ie; iron. Swinging fast is going to increase the probability that your detector will miss the good target because your swing speed is faster than your machines speed at which it processes targets.

Edit: I dont know anything about your machine. Try air testing while using a good and bad target. Put the good in front of the bad then vice versa. Then try putting 1 in between your middle and pointer and the other in between your middle and ring finger and test. Also try bad target front good target a few inches behind. Ive gotten strange results when air testing my f2 this way.

There is an optimum sweep speed for every detector , some faster and some slower. But usually if you swing too fast even though you can sometimes get better depth that way , the detectors discrimination may not work properly and your target ID meter if you have one will usually not be accurate. Too fast also increases the chances of falsing.