JSON serialization and deserialization has seen significant performance degradation since the switch over to QCbor in Qt 5.15.0.  My benchmarks show it is 2x slower on average for deserialization, yet JSON serialization is even worse than 2x slower quite often.

It appears Qt 5.14.x and earlier, which used the original Json serializer/deserializer was much faster.

Since JSON may be used in heavy client/server applications, such a performance hit is not optimal.

Suggest: See if you can make the new JSON backend performance be on-par with previous Qt versions.