Npgsql is a great connector. Just upgrade to a new one. Make sure you look at the documentation, this is really good. This will solve the speed problem.
You asked about an alternative, so I should also recommend another good connector: dotConnect for PostgreSQL. This is made by Devart. There is a simple free as well as fully reliable payment connector. The payment has Linq and entity support. http://www.devart.com/dotconnect/postgresql/
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