If your are using MySQL, you can use NHibernate, Enterprise Library or iBatis, because LINQ to SQL and Entity Framework do not support MySQL database.
The performace of NHibernate is base on your design of data models, for example, you can reduce some relationships between data models. But, base on my experiences, using NHibernate can really really improve your performance of development speed!
At last, I want to say, your success of the project is not only base on your design of data access layer, but also heavyly rely on the design of business layer, database architecture and other aspects. Relax yourself and have a good work style!