If On Premise, I wonder if your slow performance is due to the machine, the Adobe Campaign configuration, the internal network bandwith or whatsoever.could you manage some CURL script or SOAP UI or Postman tool on the server in order to check response time?
So you could know if it comes from Adobe Campaign or not.
Do you use a proxy server or direct internet connection?
Workaround-wise, HTTPClientRequest is just a reduced set of libcurl functions. You can call curl (or your own app) from execCommand(); I've made drop-in replacement libs in the past to do this type of work, usually to fix header behavior.