Unity scene increased Size and slowdown

This morning when opening my scene the custom UI reverted back to the default.
I then noticed that when pressing Play in the editor it takes about 10 sec before the scene loads (it has always been immediate) and the same when Stopping.

Finally I just noticed that the .unity file for that scene is now 100Mb (as opposed to the 600kb that it used to be).

Also now deleting an entity from the scene always takes about 5 secs (used to be immediate).

I also tested by opening a different project and it runs smoothly.
I tried going back on the script changes I did but the problem is still happening. Somehow this scene file changed on restart of Unity and now its slowing everything down.

Even if I make a brand new clean scene on the project it still takes about 10 sec just to play it in the editor (with no scripts or gameObjects in the scene).

Any suggestion? Its getting me worried!

Ok, so I realized what the issue was, I have a class that contains other classes inside but I don’t understand why its an issue.

I have this class:

class ScriptStep {
    var stepType : StepType;
    var subScript : List.<ScriptStep>;
    var stepDuration : float;  
    var stepTransform : Transform;
    var audioClip : AudioClip;
    var textLine : String;
    var stepAction : function():void;
    var stepActionFade : function(float):void;
    var stepSelectedObject : Obj_Properties_Class;
    var stepPlayerTask : PlayerTaskList;
    var stepWifeTask : WifeTaskList;
    var stepBurglar01Task : Burglar01TaskList;
    var cooperateScript : List.<ScriptStep>;
    var refuseScript : List.<ScriptStep>;
}

and if I comment out the last 2 variables (cooperateScript and refuseScript) the file size is 800k, if I add them then the file size becomes 80 Mb and I get all the issues mentioned above.

Would love to understand the issue before being able to mark it as resolved.