Don't throw ArgumentNullException in overloaded functions that merely forward the call to the "real" function with additional default parameters.
This prevents inlining of otherwise perfect inline candidates and doesn't add any value/information because the "real" function throws the ArgumentNullException anyway.

For example: Certificate.CreateFromCerFile(byte[] file) throws ArgumentNullException and Certificate.CreateFromCerFile(byte[] file, int offset, int size) also throws ArgumentNullException.

Sorry, this was a bad example...

SecureSocket.Send(byte[] buffer, int size, SocketFlags socketFlags) and SecureSocket.Send(byte[] buffer, int offset, int size, SocketFlags socketFlags) is an example where the former function doesn't need to throw ArgumentNullException.

As you probably noticed after you wrote your first post, most of these parameters checks are necessary to avoid a null reference exceptions.
As far as I can see are SecureSocket.Send with that particular signature and SecureSocket.Receive with the same signature the only two methods that should be modified.

Also keep in mind that
1) both methods are virtual, so they will never be inlined
2) a block of code that throws exceptions can still be inlined, as long as there are no catch blocks

Yeah, it turns out these two were the only ones - somehow I happened to look right at these functions first :D

Some other things that should improve performance: using Buffer.BlockCopy instead of Array.Copy; encapsulating handles in small leaf nodes (to avoid promoting large objects because of finalization) and replacing finalizers with dispose methods that close the handles to allow deterministic closing of handles and avoid finalizitaion (for example CertificateStore class could use this change)

I noticed that pinned GCHandles and Marshal.Copy are used in places where Buffer.BlockCopy would suffice.
For example in RIPEMD160Managed.HashCore function.