If you get Access Denied then, I do not know what's the problem.
Anyways here's how to do it, You may also want to do this as well to BlackCipher.aes set it to 1 core. You may not have 6 cores, I have 6 because I'm using AMD Bulldozer which has 6.
Open up Task Manager (CTRL+ALT+DEL)
Find MapleStory.exe, then right click it.
If you see all your cores checked then untick to 1 or the lowest possible. (Note: you may not have 6! This depends on how many cores you have.)
Now untick to 1 or 2 if possible then click OK. and also you may do this to many other unessential programs that are using CPU usage set it to the lowest core, For extra CPU speed do the same for BlackCipher.aes