from https://forum.qt.io/topic/67632/slow-bluetooth-low-energy-device-discovery-and-other-ble-issues which describes the issue/feature request quite good:
The QBluetoothDeviceDiscoveryAgent seems to only be using the heavyweight 12-second android api startDiscovery() scan, as it may find both BT classic and LE devices, although that call is only meant for BT classic devices. The only option LimitedInquiry does not set BLE mode. For BLE, android api startLeScan() (api level 18) or startScan() (api level 21) should have been used. These calls are much faster and scanning time can be set.