We maintain a list of external benchmark results here people have been experiencing to provide an idea of the relative performance you can expect from real-world usage:
EF & JSON.NET 2300ms EF [AsNoTracking] & JSON.NET 973ms EF [AsNoTracking] & ServiceStack 809ms Simple.Data & JSON.NET 1598ms Simple.Data & ServiceStack 933ms ServiceStack.OrmLite & JSON.NET 405ms ServiceStack.OrmLite & ServiceStack 245ms
ServiceStack 9667ms WebApi 30407ms
GitHub project for the benchmarks are at: https://github.com/anilmujagic/ServiceBenchmark
ServiceStack 19s WCF Data Services (Optimized) 28s WCF Data Services 48s
One thing that I was completely surprised by today was the performance of the open source ServiceStack.NET frameworks. Even with the performance tuned release of JSON.NET I found that ServiceStack.Text beat it by a rediculous margin. It serialises 2x faster and deserialises 4x faster.
Redis (Cygwin) 254ms Redis (Cygwin + fsync) 543ms Raven DB 2983ms
We've also previously graphed comprehensive 3rd Party benchmarks and a couple of our own at: servicestack.net/benchmarks/.