Sending large messages (>1MB) over TLS

Hello,
I stumbled on a performance issue with sending messages of large size (> 10 MB) over TLS connection.
For messages of up to 200 KB time for sending them over TLS is comparable with time of sending them over regular TCP/IP (TLS is around 2 times slower)
For messages of size 1 MB TLS is 10 times slower than regular TCP/IP,
and for 10 MB it is already 1000 time slower (in my case: 16 seconds vs 14 milliseconds)

Is there a library limitation that it is not designed to work with such big message sizes?

[log file](https://github.com/eclipse/paho.mqtt.cpp/files/6420581/sending_10Mb_tls.log)
[code for publisher](https://github.com/eclipse/paho.mqtt.cpp/files/6420588/main.txt)

Start by reviewing the publisher code in main.txt and reproducing the reported size-dependent timings over TLS and regular TCP/IP. Compare the results with sending_10Mb_tls.log, then determine whether the slowdown is a library limitation or an actionable defect; done means documenting the cause and an agreed fix or limitation.

Written by the indexing model from the issue text.