taking 2 much time to execution files read write opeartions

Question

// When put big file it taking  too much time to execution 

problem is here lines.Add(line);

  

 

List<string> lines = new List<string>();

string line = null;

bool success = false;

while ((line = file.ReadLine()) != null)

{

lines.Add(line);

                               

success = true;

}