I think Go is almost the ideal example here, you're right. Go provides a pretty rich "standard library" for writing web serving stuff so it's a good place where you could really imagine writing your performance critical stuff just on the base platform even if you use something like Webgo for the rest of your app.

Some of the other platforms are much less amenable to that since the standard primitives the platform exposes are very primitive indeed (servlets api, rack api, etc.). Perhaps there's some value in looking at how your favorite framework stacks up against its raw platform and trying to contribute some optimizations to close the gap a bit.