I see, you likely need a weak Key concurrent map, but that's going to introduce some overhead of its own.

Right, even though there is actually already some caching going on in this class. It uses a ConcurrentMap in this case. We probably should have a look at this as well. Either way, this does not cover the caching of the reflection call Class#getInterfaces. Thinking about it, much could be gained by adding a special case for the default group (java.validation.Default). In this case there is no need to call #getInterfaces at all and I would assume that is the most common case. Might be the reason why you noticed the contention in the first place. Probably a lot of parallel validations of the default group leading to contention around the lock for Class#getInterfaces.

What about using the Jandex index to read these?

I don't think this would make much of a difference and I don't want to introduce another dependency just for that.