Slow appenders cause Logback to be a significant bottleneck in the system. AppenderBase doAppend is synchronized to allow Appenders to be threadsafe. This is an extremely poor way to accomplish that. Many Appenders are slow since they are dealing with I/O of some kind but may be thread safe. Where they are not they should deal with the issue at the smallest granularity possible. AppenderBase must be changed to not be synchronized and any Appenders that are not thread safe should be modified appropriately.