ToList() creates a new List object and copies all the elements from the source IEnumerable<T> into the new List. This means that it has to iterate through all the elements in the source IEnumerable<T>, which has a time complexity of O(n), where n is the number of elements in the source IEnumerable<T>.
This can have a performance impact, especially if the source IEnumerable<T> is large and the operation is being performed frequently. In these cases, it may be more efficient to use a different approach, such as using a loop to add the elements to the List manually, or using a different data structure that allows for more efficient insertion and retrieval of elements.
However, in many cases the performance impact of ToList() will not be significant, especially if the source IEnumerable<T> is small or the operation is only being performed infrequently. It is generally a good idea to use ToList() when you need to work with a List rather than an IEnumerable<T>, as it can make the code more readable and easier to work with.
