C++ - Read log level in performance oriented way

The logger component is a static library. Application (service) loads around 20 DLLs. Application and DLLs are linked with the static logger library. There are other applications (UI/CLI) that update the log level in the shared memory on Windows.

Main application and DLLs read log level from the shared memory and decide whether to log or not a particular log statement. Reading log level is protected by the named mutex which has a huge performance penalty.

What can be the best way to make the reading log level lightweight.