WPE is a packet editor, it basically modifies packets that are sent to the server and received from the server.
WPE is illegal on almost every server, however there are just a few admins out there that are able to detect and protect against WPE.
Players could just have skill, imagine a server without delay, the speed of your casting is mostly exactly the speed of your ability to spam hotkeys (plus network delay etc.)
The computer specs can do some stuff, if you lag you will be slower in casting than people that don't lag.
Put your hotkeys right, as for a champ on a high rate server I put them like
F1 = Ygg. Berry
F2 = Zen/Dangerous Soul Collect
F3 = Fury/Critical Explosion
F4 = Asura Strike
With this setup, you just need one hand on those 4 keys and some training, the other hand on your mouse.
F5 should be Snap/Body Relocation then and F6 Blade Stop or Palm Push Strike, whatever you prefer.
WPE/RPE or simple macros are possible, though, but honestly, I can spam those keys as well, easily, it's just about setup and training.