I do ippsRSA_Decrypt in tow threads running in different CPU core at same time, it will cost more time than in only one thread.
I linked the single-thread lib, it is thread-safe and none nested parallelization, but why efficiency is degraded in this case?
IPP is threaded safe, and for this function, it is not expected this function in one threaded will impact another threading. For your case, have you got chance to check the synchronization at the application level, and see it works well?
Also, you could submit one test code at the support website to Intel IPP product: https://software.intel.com/sites/default/files/managed/41/2a/SubmittingSupportIssue.pdf
Our support expert may have further check as well.