Tongue in cheek aside - this would be really cool if it actually ran in a browser. Linking to external content, having resource based URLs, etc - all the good things about web development - are taken away once you leave the browser.
Windows perf suffers because (1) in some rare cases, the needed primitives aren't provided by the kernel, and (2) Windows does some things differently, and Ruby library authors don't have the incentives to design and maintain a separate codepath for Windows.
The issues above don't usually arise in desktop applications. They're seen most often in server code -- fork is missing and that sucks, asynchronous I/O works differently, file monitoring and pipes also work differently, etc.