There's code to compute the value of "year 0" in all the Mozilla date tests. There's no need to compute this on entry to each test. This is not all of why the tests are slow, but it's at least part of it.
Created attachment 3271 [details] patch to use "year 0" constant instead of computing it each time
Comment on attachment 3271 [details] patch to use "year 0" constant instead of computing it each time looks good
I'm pretty sure the rest of the slowness can be corrected by similar fixes -- turning repeated calculations into constants. The date tests are pretty pedantic in how they calculate correct answers.