During performance analysis of our application the LogbackMDCAdapter showed up as a performance hotspot. This is because the application does relatively often replaces multiple entries in the MDC at ones, e.g. 6 removes/writes without any intermediate log statement. Also actual log messages that come through filtering before creating the LoggingEvent are relatively rare in production environments.

I have reworked the implementation to improve the performance. The main idea is to defer cloning the internal Map as long possible. This patch increased overall application performance by about 10% in that test.
Other application types were MDC changes are small and many LoggingEvent objects are created (calls to getPropertyMap()) should not be affected significantly.

Note: I haven't check if my patch would reintroduce the #LBCLASSIC-183, but my subclasses of InheritableThreadLocal does not override the initialValue() method, so it could work.