Based on the "log4j performance comparison" discussion in the logback-user mailing list:
http://old.nabble.com/log4j-performance-comparison-td31183677.html

I'd like to see at least the ImmediateFlush option back as it causes really big difference in writing performance at our systems with heavy logging activity.
Last logback release with ImmediateFlush option is 0.9.18 and this is the one we have to stay at due to performance reasons. We can't afford to upgrade to later logback release or switch ImmediateFlush to true as it makes some of our systems practically unusable.

The best option seems to be the use of FileWriter with the possibility to set the ImmediateFlush option and also buffering (wrapping with BufferedWriter and use of BufferedIO + BufferSize options) can help a little.