[logback-user] appenders

Hi, I have migrated from log4j to slf4j and logback. I have one question for the appender setting. In log4j, you can set the threshold of an appender so that only a certain level of log messages will be logged by that appender. But in logback, I can only achieve this behavior by using a expression filter. I would like to know the performance figure about that filter. If the expression filter takes much longer than a dedicated log level filter, I would like to see a simple log level filter be included in logback. Thanks, Eric