Could you please specify what is the system memory and what is your startup arguments in terms of JVM heap size and number of
thread WLS is using. Fine tunning these 2 parameter are very important for performance. Increase thread (say if it 15 make it 30) and give JVM extra memory as specified by WLs6.1 docs. Also other parameter for connection pool, initial size and increment size.
We use WLS6.1 on SunSolaris and tunning above parameter helped server performance.