I have used the Avantree Pro Bluetooth transmitter and receiver set for some time. They work great. The delay is 32 milliseconds (.032 sec). If you are swinging the coil at three feet per second you will hear the target (36” x .032) = 1.152 inches after you cross over it. When moving the coil slowing while pinpointing the delay is insignificant.
If you purchase these be sure you are ordering the Low Latency Pro model. They have a cheaper set that has an unacceptable delay time.
Have a good day,
Chet