KAL is an Intel Pentium 4 clocked at 2.53 GHz running XP and Sun's JDK 1.4.1.

Results are in microseconds.

NullAppender:

NullAppender:

    4.11 PatternLayout "%p - %m%n"
    4.11 PatternLayout "%-5p - %m%n"
   13.52 PatternLayout "%r [%t] %-5p %c{2} %x - %m%n"
   11.02 PatternLayout "%r [%t] %-5p %.10c %x - %m%n"
   11.72 PatternLayout "%r [%t] %-5p %.20c %x - %m%n"
   10.01 PatternLayout "%r [%t] %-5p %c - %m%n"
   18.53 PatternLayout "%d{ISO8601} [%t] %-5p %c %x - %m%n"
   18.83 PatternLayout "%d{yyyy-MM-dd HH:mm:ss,SSS} [%t] %-5p %c %x - %m%n"
  127.18 PatternLayout "%l - %m%n"
  142.61 PatternLayout "%C.%M.%L - %m%n"

FileAppender:

   10.82 PatternLayout "%p - %m%n"					   
   11.82 PatternLayout "%-5p - %m%n"					   
   20.53 PatternLayout "%r [%t] %-5p %c{2} %x - %m%n"			   
   19.03 PatternLayout "%r [%t] %-5p %.10c %x - %m%n"			   
   19.63 PatternLayout "%r [%t] %-5p %.20c %x - %m%n"			   
   17.53 PatternLayout "%r [%t] %-5p %c - %m%n"				   
   28.74 PatternLayout "%d{ISO8601} [%t] %-5p %c %x - %m%n"		   
   29.14 PatternLayout "%d{yyyy-MM-dd HH:mm:ss,SSS} [%t] %-5p %c %x - %m%n"
  162.13 PatternLayout "%l - %m%n"					   
  198.69 PatternLayout "%C.%M.%L - %m%n"                                     

FileAppender: (ImmediateFlush=false)

   6.31  PatternLayout "%p - %m%n"					   
   6.01  PatternLayout "%-5p - %m%n"					   
  14.42  PatternLayout "%r [%t] %-5p %c{2} %x - %m%n"			   
  14.22  PatternLayout "%r [%t] %-5p %.10c %x - %m%n"			   
  14.42  PatternLayout "%r [%t] %-5p %.20c %x - %m%n"			   
  13.02  PatternLayout "%r [%t] %-5p %c - %m%n"				   
  23.13  PatternLayout "%d{ISO8601} [%t] %-5p %c %x - %m%n"		   
  23.53  PatternLayout "%d{yyyy-MM-dd HH:mm:ss,SSS} [%t] %-5p %c %x - %m%n"
 147.51  PatternLayout "%l - %m%n"					   
 166.04  PatternLayout "%C.%M.%L - %m%n"                                     

FileAppender: (BufferedIO=true)

   5.81 PatternLayout "%p - %m%n"					   
   5.81 PatternLayout "%-5p - %m%n"					   
  14.12 PatternLayout "%r [%t] %-5p %c{2} %x - %m%n"			   
  14.22 PatternLayout "%r [%t] %-5p %.10c %x - %m%n"			   
  13.92 PatternLayout "%r [%t] %-5p %.20c %x - %m%n"			   
  12.82 PatternLayout "%r [%t] %-5p %c - %m%n"				   
  22.73 PatternLayout "%d{ISO8601} [%t] %-5p %c %x - %m%n"		   
  23.23 PatternLayout "%d{yyyy-MM-dd HH:mm:ss,SSS} [%t] %-5p %c %x - %m%n"
 149.73 PatternLayout "%l - %m%n"					   
 164.46 PatternLayout "%C.%M.%L - %m%n"                                     

Notice the small but noticable performance gain when
ImmediateFlush=false

Note also that BufferedIO yeilds similar performance to the
ImmediateFlush case.