#1: When you can see more, you can design more.
Kumar Shah discovered an exploit in the editor where you can zoom out more. Heres how you do it:

-Go to the cinematic editor.
-Click new track
-Zoom in about half as much as you can. Click new waypoint
-Zoom in as far as you can, click new waypoint.
-On the second waypoint (the one that's at the middle) set the tension to anything over 25.
-Set a duration to the camera track and click play
-Slide the progress scroller to "select" your desired zoom level.
Now you can see alot more, and ofcourse design faster.
(note that you'll get some lag and chopiness if you zoom out too far or have a slower graphics card. If you want to zoom out really far set the graphics setting on low. You'll get alot less lag)

Pic

#2 Use Hotkeys

Hotkeys let you jump to commands or actions in the editor just by pressing a key on the keyboard.
Press W for example will open up the water menu. Theres alot of hotkeys:

-Take a look at This Hotkey List Article and study the commands. After using them once or twice you should memorize them and it'll become second nature to press "[" to switch the brush to a circle or "]" to switch it to a square, or pressing 1-0 on the keyboard to change the brush size, or pressing ctrl+C to copy something or ctrl+V to paste, or ctrl+f1 - f8 to switch between players when placing things.
The list goes on

#3 A tip on quickly changing the brush size

While the painter is on the screen, hold down ctrl, then hold down the left mouse button and scroll up or down to adjust the brush size. Up makes it bigger, down makes it smaller. This changes the brush size in seconds, and can make it bigger than the maximum 20 tile radius (you can make it as big as you want actually)

#4 Don't obsess on one part of you map
"polishing" ,as I call it, in AOM takes alot of time because of the so many different variables to manage. Theres terrain, elevation, lighting, cinematics, triggers, units, objects, etc etc. People seem to like to prod and poke at things. Don't do that, people will likely not notice the change So design your terrain, adjust elevation, plop some units down, and rotate a few, delete a tree or two and move on. A mistake I made when working on my world map is that I'd spend too much time poking the elevation, and in the end it really didn't look too different from what it originally looked like.

Not everyone "over-polishes", but for those that do - you'll go much faster and feel like you've done so much more if you don't

#5 Ask people/friends for input
When you have a clear idea what to make, and even suggestions and examples you'll design much faster. Why? because thats how it works. I recommend asking someone their opinion on a part of your map you're working on, and shape it out.

#6 Have a clear idea what you want to do
Most designers don't think of an idea for a scenario when they're just messing around in the editor, but rather when they get an idea or inspiration from something else, whatever it be. Thats why it's also a good idea to get ideas and think about how you want your map to look like, how your cinematic will roll out, and so on. When you go to do it in the editor you'll do it much faster than as if you were brainstorming for ideas on what to place next.

#7 Develop "designer" habbits
You can't really learn this, however I'll explain what it is. Everyone has their own style in designing. Some make very long cinematics that just pan around their map so that they can inspect their map design, others will frequently test the map, and so on. Developing these 'habbits' will give your scenario their own unique thing going on. It'll be different from others, and in a scenarios gameplay you can even sometimes see the style in which a designer designed the scenario, like if he was designing while pitching in.

Also having sort of "order builds" when making your scenario could help. Like placing elevation, then terrain, then trees, units and buildings... In the end these "habbits" will familiarize you with the editor more, you'll design faster, and have a kind of unique design to your scenario.

the AOM editor is so different from the AOK one, theres very little "boundaries", you're not limited to placing units on tiles, you can make your own water, triggers etc. I think a scenario will reflect alot on the person and the way he designed it.

The #1 most important thing that will help you design faster and better is simply expiriance. I've been designing since AOK, so I know my way around so to speak. Exploring the options and features and testing them out is a big way to get expirianced. You probably would learn the most from making and completing a scenario. Even if it sucks, you'll learn alot and atleast know how to make another scenario.

I'll add more things to this guide when I get more ideas

Thanks for reading