The de.thetaphi:forbiddenapis Maven plugin provides some good verifications against usage of dangerous APIs like sun.misc.Unsafe and methods which have a different semantics depending on system locale, local timezones, etc..
Is that optional as not to slow down the build even more?
No it's not optional.
Did you notice any slowdown? I didn't notice any, but measured now that you asked and it seems to take ~1 second more than previously.