When running some benchmarking (specifically https://github.com/stephenc/java-logging-benchmarks/blob/master/log4j-fmt/logback-async/src/main/java/me/justpitch/loggingbenchmarks/LoggingBenchmark.java - at time of issue creation @ceki has access to repo, but repo is due to become public soon) I noticed that the Async appender does not scale well when the number of threads increases.

In fact the Async appender - which to my understanding is designed to help provide a more consistent latency under multiple threads - has dire performance when the number of threads increase, e.g.

The above results show that Log4J with an async appender has a profile very similar to that of both Logback and Log4J with a non-asynchronous appender. Logback's asynchronous appender performance gets progressively worse and approximates the performance of Java Utils Logging above 16 threads on the test system