Obtaining default system ZoneId is a frequent call. It should have as little overhead as possible. This call is implemented as: TimeZone.getDefault().toZoneId(), so the opportunity is to cache the instance inside default TimeZone instance.
JDK-8084932java.time.ZoneId.systemDefault() should be faster
JDK-8074023Clock.system(ZoneId) could be optimized to always return the same clock for a given zone.