The energy from the charge pulse generated by the antenna is stored in a capacitor inside the PIT tag. Once the capacitor is charged any further time is wasted. In some situations the tag will charge in less than the default time of 50 ms.
To find the minimum charge time for a system, hold a tag at the edge of the detection field. Decrease the charge time 5 ms and see if the read range has decreased. If not, subtract another 5 ms until the tag stops working. That is the minimum time needed to charge the tag with that antenna and reader. It's a good idea to add a few milliseconds to the settings to make sure all tags are fully charged.
Once you find the optimum charge time you can adjust the listen time to set the scan speed. Increasing the listen time will slow down the scan rate to save power and increase run time from batteries.