wolfSSL uses mutexes for most locking synchronization. In release 5.6.0 we have added support for pthread_rwlock_t (https://github.com/wolfSSL/wolfssl/pull/5952 and https://github.com/wolfSSL/wolfssl/pull/6086). It is currently implemented in the session caching logic. This will speed up multi-threaded servers by allowing multiple threads to read from the cache simultaneously. We also recommend multi-threaded servers to define ENABLE_SESSION_CACHE_ROW_LOCK when building wolfSSL. This will initialize and use a separate lock for each row in the cache.
If you have questions about optimizing wolfSSL please contact us at facts@wolfssl.com.