What are the differences between using GetComponent<Transform>() and this.transform?

The Speed (fast to slow):

cached _transform >> Component.transform >> Component.GetComponent<Transform>

And If you disassemble a UnityEngine.dll C# binary, you can see transform do not call GetComponent (Component class member), just call internal mono runtime method. Here is the transform code.

// UnityEngine.Component
public Transform transform
{
    get
    {
        return this.InternalGetTransform();
    }
}

[WrapperlessIcall]
[MethodImpl(MethodImplOptions.InternalCall)]
internal extern Transform InternalGetTransform();

And Here is disassembled Component class's GetComponent Method.

public extern Component GetComponent(Type type);
public T GetComponent<T>() where T : Component
{
    return this.GetComponent(typeof(T)) as T;
}
[WrapperlessIcall]
[MethodImpl(MethodImplOptions.InternalCall)]
public extern Component[] GetComponents(Type type);

GetComponent is a kind of Finding function using generics. So it's a little bit slower than caching.

Original Source :

public class Test : MonoBehaviour
{
    Transform _transform;
    void Start()
    {
        _transform = this.transform;
        _transform = gameObject.GetComponent<Transform>();
    }
}

Binary Disassembled:

public class Test : MonoBehaviour
{
    private Transform _transform;
    private void Start()
    {
        // == UnityEngine.Component::get_transform()
        this._transform = base.get_transform(); 

        // == UnityEngine.Component::GetComponent<UnityEngine.Transform>()
        this._transform = base.get_gameObject().GetComponent<Transform>();
    }
}