set the priority of callback to high introduces some delay compared to simple MultiThreadedExecutor

hi, I try to set one callback higher priority following the example cbg_executor . But It seems it introduces some delay compared to using MultiThreadedExecutor.

I publish on the topic "high_pong" at rate 100Hz. In the pong_node.cpp, I set the parameter "high_busyloop" to 1.0, and print the timestamp when receiving the msg.
Use the code in pong.cpp (setting high_prio_executor to ThreadPriority::HIGH), the output is (msg receiving interval is 1050ms):

But if I simply use MultiThreadedExecutor and add the pong node:

  auto pong_node = std::make_shared<PongNode>();
  rclcpp::executors::MultiThreadedExecutor multi_executor1;
  multi_executor1.add_node(pong_node);
  multi_executor1.spin();
  rclcpp::shutdown();
  return 0;

the output is (msg receiving interval is 1000ms):

Start with the cbg_executor example, then compare pong_node.cpp and pong.cpp where high_busyloop is set to 1.0 and high_prio_executor uses ThreadPriority::HIGH. Reproduce the high_pong timing with both that setup and MultiThreadedExecutor, then trace the callback scheduling and thread-priority behavior to identify the source of the delay. Done means the cause is explained and the priority-based executor no longer adds the reported interval delay.