This table shows the user cpu time for 6u3 and 6u4 reported by
/bin/time for running 32 bit javac on various numbers of files on a Sun Blade 2500, 2 cpus. Sol 10.
Options used are:
   -J-XX:ThreadStackSize=768 -J-Xmx896m -J-Xms128m -J-XX:PermSize=32m -J-XX:MaxPermSize=160m

                6u3 6u4 Per cent degradation
Server:
    122 files 12.3 13.2 0.07
    203 files 13.7 15.6 0.14
    435 files 31.6 36.2 0.15
                        
Client
    203 files 5.3 5.4 0.02

I added the option
    -XX:+PrintGCDetails
and the two log files shows that the GC time is the same for 6u3 and 6u4.

I swapped libjvm.so in 6u3 and 6u4 and the degradation followed libjvm.so.

The attached script can be used to repro this problem.