Nested dictionary serialization

Hi! I want to serialize a nested dictionary in the fastest (performance) way possible. Here is my declaration:

 public class ExampleClass {
     public Dictionary<string, Dictionary<string, float>> nestedDicts = new Dictionary<string, Dictionary<string, float>>();
 }
 

It could be JSON, XML, or binary, I don't mind, as long as it is performant and platform-independent.

I am looking for something like:

 ExampleClass ec = new ExampleClass();
 //encode
 var encoded = Serialize(ec);
 //decode
 ExampleClass ec2 = Deserialize(encoded);
 

I am looking for something that comes with MIT license or similar because it will be part of an asset for the Asset Store.

I tried FastJSON but it keeps crashing Unity. Anyone knows an alternative (or how the crashes can be resolved)?