His benchmarks show that Opera Unite can do up to "impressive 800 requests per second" on decent hardware, and even with dynamic content. You also can't DDoS a person via Opera Unite. He also says that Opera Unite uses "very smart file I/O", and that even if you save data to file, you can push out 744 requests per second.
As a comparison, PHP+Apache+MySQL is almost 2 times faster, nginx (one of the fastest servers available) is only 5 times faster, and the MadFish WebToolkit ("compiled raw C++") Web server is only 6 times faster.
Still, he concludes, Unite beats them both at ease of use.
Read more: Opera Unite benchmark @ unitehowto.com