If you use a context property in your appenders pattern, there will be a significant logging slowdown. In my experience anywhere from 3 to 5 times slower (this will depend on the appender).
I believe that as soon as you use a context property log4net will internally access the windows user name even though it's not the property you want to access. This theory comes from the fact that printing all properties in the pattern (including the costly property username) compared to just printing one will slow down the logging with the same factor.
I've made a stackoverflow question with more details as well