TLS 1.3 CPU usage indeed seems to be a bit too much in many scenarios, and we do plan improvements in this area in the future. (Update: This has improved a lot in R6.0 release!)
However, at the moment, we are still busy adding some features such as OCSP that are perceived us must-have by customers using Rebex HTTPS on legacy platforms, which are currently the component's major use case.
Also, due to the fact that Rebex HTTPS is mostly targeted at legacy platforms, it's HTTP internals are mostly implemented using classic synchronous code, which is not a good match for our fully-asynchronous TLS 1.3 implementation.
Therefore, users looking for performance and scalability are advised to consider using the low-level Rebex TLS API instead and check out the HttpsGet sample app - when compiled for .NET 5.0 and executed in quiet mode, its CPU usage is considerably lower (even compared with the same app compiled for .NET 4.x).