Rest proxy producer performance dropped after upgrading to 6.2.0

I used 6.1.1 Kafka rest proxy to produce over 500k messages per second to Kafka, however, after upgrading to 6.2.0 rest proxy, performance dropped to around 300k messages per second. When I tried to find out the reason, the producer thread in rest proxy 6.2 is limiting and causing the performance degradation. Are there been any intentional changes in the rest proxy producer recently for 6.2? If so, is there any configuration I can tune to improve my producer performance?