In C#, if the class list has more elements, it slows down crying

UNI.T is programming. There's a class called MyClass in C# Declare a list, create an object with new MyClass() and put it in Add Inserting more than a thousand objects takes a lot of time. Is it supposed to be like this?Is there any other way to handle the class arrangement? The list is more convenient than the simple arrangement, but the speed is too slow. I have to put in more than a million