Comment by anomalroil

As you said: Java's Random is not meant for serious cryptographic usage, it's meant to be super fast. You have SecureRandom instead for cryptographic usage, and it's noticeably slower. Interestingly, using fast, but bad random generators such as Java's Random was shown to be an issue multiple times in the past already for things such as Monte Carlo, and so on, not just for things related to security.