Go is much better at responding with large amounts of data when the database call is eliminated. I can only assume its ability to deal with large strings is leaps and bounds superior to v8 which makes sense to me.
In my own micro-benchmark, I've seen that BSON decoding takes big chunk of time when using mgo. That should explain why the go version got faster when DB access was removed.
This shouldn't come as a surprise as the node.js driver uses a BSON parser written in C++ by default.