High latency of downlinks in Class B from ChirpStack

I recently purchased the following LoRaWAN Gateway Kit:
I performed several tests with this kit and I also installed within the Raspberry PI 3 the ChirpStack open-source LoRaWAN Network Server stack.
One of my goals is to test the LoRaWAN Class B mode with some end nodes.
Obviously, I connected the Raspberry PI 3 to a GPS module and I also connected the PPS output to the pin 24 of the RHF0M301 module.
I used the evaluation kit EVK-M8GZOE as GPS module (https://www.u-blox.com/sites/default/files/EVK-M8GZOE_UserGuide_(UBX-16030132).pdf).
I also installed and configured the I-CUBE-LRWAN library (v1.3.1) within the end nodes.
As end nodes, I use the STM32L0 B-L072Z-LRWAN1 boards.
Currently, I tested the Class B mode with a single board; the end node is able to switch into Class B mode and to set up ping slots correctly.
Furthermore, the gateway queues and sends beacons successfully.
However, I can’t figure out why the downlinks sent by the MQTT broker within the gateway take 6/7 seconds to get to the end node despite I selected a ping-slot periodicity of 1 second.
My aim is to receive a downlink in Class B within 1 /1,5 seconds after sending the packet from the MQTT broker of the ChirpStack gateway.
I can’t use the Class C because of the power consumption.

For what concerns the options in the Device Profile Class-B section, I choosed:

  • Class-B confirmed downlink timeout: 1 second
  • Class-B ping-slot periodicity: every second
  • Class-B ping-slot data-rate: 3
  • Class-B ping-slot frequency (Hz): 869525000

Is it possible to reduce the downlink delay of 6/7 seconds in class B mode? If so, how?
Waiting for a kind reply.

Best regards