Generally the existing javascript bitcoin miners now get around 0.1MH/s if using multiple threads. Given that a large number of machines ( website users ) can potentially do a lot of computation, I have started working on a javascript miner ( not java ) a few months ago which can take advantage of just-in-time compilation in the new browsers to achieve a high speedup compared to the existing javascript miners. I performed a lot of code optimization techniques and my preliminary results show more than 10X speed up compared to the best code that I found in internet. This will allow reaching a performance competitive with what can be achieved while running C/Java codes. Please message me if you think this is something that might interest you in any way or if you have any insights.