If your application is desktop or client-server this is the worst idea which could come in head of your management.

The idea that .NET isn't suitable for desktop and client-server apps is a myth. I've been involved in multiple apps along these lines, and I disagree with this statement.

According database C# application work 3-10 times slowly.

A month ago, it was 5-8 times. I believe I asked you then to qualify those statements.

This just doesn't need to be that way. Excluding 'smaller' database apps, effective construction of queries and stored procs in the back-end can lead to a .NET app with comparable speeds in the critical processing areas...and sometimes faster as the database and other factors grow.

Kevin