How sealed Class Improve performance

Incidentally, unless you're writing a performance critical application, I wouldn't worry about making your classes sealed. If the C# language designers had thought this important, they'd probably have made classes sealed by default.

If you're writing a library for others to use, then a more important consideration is whether it's a good idea to allow them to inherit from your classes - sometimes it isn't and you'll therefore find that some classes in the .NET framework itself are sealed.

As well as making a whole class sealed, you can also seal an 'override' method in isolation to prevent further overriding. Again this would usually be done for design rather than performance reasons.

It's still virtual because if Derived were not sealed then MyMethod() could be overridden again by another class which derives from Derived.

You're right that a sealed class can't contain new virtual methods, but it can contain overrides of virtual methods which are still virtual.

For example the following compiles fine: