Entity framework vs NHibernate - Performance

Question

I am looking to implement an ORM into our system. We currently have many tables with lots of horrible data and stored procedures. I've heard using an ORM can slow the system down. Does anyone know which ORM is better on speed and performance using Queries created in the C# code and mapping to stored procedures?

Thanks

EDIT:

The project will use existing tables that are large and contain a lot of data, it will also use existing stored procedures that carry out complex tasks in a SQL Server DB. The ORM must be able to carry out transactions and have high performance when running the existing stored procedures and querying the current tables. The project is web based and will use WCF web services with DDD. I can see that EF is a lot easier to use and has greater support but if NH the most suitable option?

Answer 1

check this artical Some Advantages of NH over EF

1. NH has 10+ id generator strategies (IDENTITY, sequence, HiLo, manual, increment, several GUIDs, etc), EF only has manual or SQL Server's IDENTITY;
2. NH has lazy property support (note: not entities, this is for string, XML, etc properties), EF doesn't;
3. NH has second level cache support (and, believe me, enterprise developers are using it) and EF doesn't;
4. NH has support for custom types, even complex, with "virtual" properties, which includes querying for these virtual properties, EF doesn't;
5. NH has formula properties, which can be any SQL, EF doesn't;
6. NH has automatic filters for entities and collections, EF doesn't;
7. NH supports collections of primitive types (strings, integers, etc) as well as components (complex types without identity), EF doesn't;
8. NH supports 6 kinds of collections (list, set, bag, map, array, id array), EF only supports one;
9. NH includes a proxy generator that can be used to customize the generated proxies, EF doesn't allow that;
10. NH has 3 mapping options (.HBM.XML, by code, by attributes) and EF only two (by attributes, by code);
11. NH allows query and insertion batching (this is because EF only really supports IDENTITY keys), EF doesn't;
12. NH has several optimistic control strategies (column on the db, including Oracle's ORA_ROWSCN, timestamp, integer version, all, dirty columns), EF only supports SQL Server' TIMESTAMP or all