You're showing the sum of all times, but do the times at least look better after the first run (with -Xshareclasses anyway) ?
Depending on how smoothly your experimentation went, you may also have to destroy a pre-existing cache before you really measure it (java -Xshareclasses:destroy) as it could have stale classes in it from an earlier run.
Would you be willing to open an issue with more details at http://github.com/eclipse/openj9/issues so we can look into it?
# time for i in `seq 1 10`; do ./jdk-9+181/bin/java -client -Xquickstart -jar l2i-0.1.0-SNAPSHOT-standalone.jar; done
real 0m18.571s user 0m30.429s sys 0m1.374s
# time for i in `seq 1 10`; do ./jdk-9+181/bin/java -client -Xshareclasses -jar l2i-0.1.0-SNAPSHOT-standalone.jar; done
real 0m16.642s user 0m19.483s sys 0m6.307s