for(int i=0;i<myList.Count;i++) await FileIO.AppendTextAsync(myFileName, myList[i]);
The method is of course marked as async, so when my debugger skipped over that line and nothing happened, I though something was wrong with Visual Studio. Then I set the debugger at my AppendTextAsync line and when it stopped, I realized that it was taking forever for the computer to append the items from my list, one-by-one (there were more than ten thousand items, and my computer is not the slowest one out there.
string result = string.Join(Environment.NewLine, myList.ToArray());and voila! It took a split second.