The performance of java.lang.Class.isAssignable api in SUN's jdk1.5.0_06 build 05 lags behind the peformance of the same in JRockit's jdk1.5.0_06.
The JRockit api takes 50% (or half) of the time that SUN's jdk1.5.0_06 takes, after the "warm-up". This was measured using a simple micro-benchmark that repeatedly called isAssignableFrom.
The measurement was done on Solaris 10 on SPARC.
Right now, I am filing this bug so that it's on the radar. I will be digging deeper for more details.