WebRender has matured a lot lately; it's even in Firefox nightlies behind an about:config preference. That said, there are still rough edges. If you see rendering slowness, feel free to file issues on http://github.com/servo/webrender.
There are no silver bullets for performance, only lots of lead bullets. I'm confident that parallelism via GPU and multicore is a large architectural win. But lots of making the Web fast involves a ton of little special case optimizations that just need to be implemented. That's how Stylo went: promising benchmarks and a mediocre user experience at the research stage turned into great benchmarks and a great user experience with production engineering work.
There are no silver bullets for performance, only lots of lead bullets. I'm confident that parallelism via GPU and multicore is a large architectural win. But lots of making the Web fast involves a ton of little special case optimizations that just need to be implemented. That's how Stylo went: promising benchmarks and a mediocre user experience at the research stage turned into great benchmarks and a great user experience with production engineering work.