Looking at a presentation for StackOverflow, I got curious on the in-house ORM they're using: it's called Dapper .NET and the say it's FAST!
https://github.com/SamSaffron/dapper-dot-net
They say it's almost as fast as hand-coded SqlDataReader, so this got my attention.
What really stands out is that EntityFramework, the popular ORM framework from Microsoft takes the last place, with over 10x slower execution time!
NHibernate, another popular ORM, is not doing that bad, but it's still 2x slower! It's definately worth a look.