Synchronized is redundant, commited my mistake.
It's about 30% slower than non-synchronized version.

Thanks, put Test.assertEquals() inside a condition, works fine.

If I put Test.assertEquals() put inside a condition I get Max Buffer Size Reached

Test.assertEquals() will output a billion messages affecting time of execution

Added random tests

Your solution is just a little bit slower than the fastest one, so it should pass.

Thanks for your feedback, indeed these restrictions are redundant, removed them.

Thanks for the feedback, it seems there are no ways to implement fast working solution for such big numbers.
I added required range of input numbers to the description.

It seems numWays method is broken in random tests. I tried a few working solutions from similar kata https://www.codewars.com/kata/counting-change-combinations/ and they all fail on some random tests here.
For example:
cents = new Currency([ 145, 27, 11, 9, 1 ]);
Test.assertEquals(cents.numWays(195), 676, "random test did't accept 676 here");

Very elegant solution, but it can cause stackoverflow with long lists.