I'm sure it's true for that specific situation.
For one thing, Java *is* slower -- despite some claims to the contrary. It simply *has* to be: after all, it's not fully compiled code.
Also, for those who are not familiar with the AS/400, they might have had trouble maintaining it and tuning it properly.
I don't know about DB2/400, although UDB (DB2/2) is a very good db.