The performance tests show a bit of a performance regression when this change went in. There are a few things we can do to tweak the implementation.
I think the main problem will be using java serialization for serialising the ids. We don't necessarily need to use the original test ids, we just need an id for each test. So, I'd assign each TestMethodResult a long id and use that instead of the original id.
I'd do the same thing for test classes, too.