You can also add notes into your code to help you remember why you did what you did. Also, having groups and event sheets doesn't slow anything down.
The most important thing to remember when organizing and moving your events around and into groups is that the order of everything does matter.
SO if you have a bunch of event sheets and move one above or below where it was, things might stop working in your game. Not a big a deal, you can move it back-- but if you don't realize this is the issue right away, you could end up debugging for a long time.