The binscripts build target takes around 3 seconds in the environments I use for building Derby. It looks like most of the time is spent running the chmod Ant task on the generated scripts. There are six invocations of the chmod task, one for each script. The chmod task can operate on multiple files, so one invocation should be enough (and one invocation on six files seems to have about the same cost as one invocation on a single file).
A related problem is that the scripts are always regenerated in incremental builds, even when there's nothing to do, so the binscripts target accounts for a significant part of the time it takes to run an incremental build.