Pulsar C++/Python consumer performance issue, if producing message in batch mode, consuming ACK in single mode #3623

Expected behavior

Pulsar C++/Python client consuming performance should keep at a stable level;

Actual behavior

Pulsar C++/Python client consuming performance downgrade obviously;

Steps to reproduce

1. Producing messages in batch mode (locally with batch count as 1000);
2. Consuming messages with ACK in single mode (as appose to the cumulative mode) with C++ or Python clients;
3. Keep the consuming process running for a while (say, with producing message 5000-8000 per second, run at least 15 minutes) and see the consuming messages per seconds, it downgrades very obviously; At the same time, the Pulsar Java client consuming performance could keep in a stable level.
   See the attached monitor screen below:
   

System configuration

org.apache.pulsar:pulsar-client-cpp: 2.2.0