It is difficult to test JS workers performance:
1. In Firefox 24 a bit heavier worker with the boundary tracing algorithm is 1.5 times slower than the same script without workers yet.
2. In Firefox 26 (Alpha) both tests have the same 1.5 times slower performance (shall I test Beta? :)
3. In Chrome 29 and 32 both tests have similar performance (a bit slower then in Firefox) and I can't test workers off-line in Chrome.