How do we know that it really took Palpatine that long to master the technique once he discovered it, though? Maybe it's easier than we think. After all, the dark side is said to be the quicker and easier path to everything.
Use your head. That's how it works when it comes to really complicated Force processes. Considering that your Force ghost gets sent to Force hell if the attempt fails, you damn well better have spent a long time researching and ****. Furthermore, your quotation about the dark side making things easier is taken out of its proper context. In proper context, it means that Jedi turn to the dark side for power for easier solutions to problems, nothing to do with making it faster
to learn those powers. Besides, even if it was easier to learn than we think (which we have no reason to believe), when would the apprentice have ever had learned that it existed, let alone have the time
to figure it out?