The game runs slowly when I test my level
Unfortunately it can take a pretty powerful machine to get an emulator to run at full speed in a browser.
Try using Chrome as your browser, as it runs the emulator the fastest.
There isn't much that can be done about this for the time being I am sorry to say. But there are some possibilities coming in the future that might help 🤞
Some weird glitch is happening in my level
This might be a bug in Smaghetti, or it could be a bug in Super Mario Advance. Glitches are going to happen sometimes, just the nature of making a level editor like this.
Also check for warnings
Smaghetti can often detect if your level is going to have a problem. When it does, it will put this symbolon an entity that is causing problems. Click on that symbol to see what can be done to fix the problem.
Why do so many things get grayed out in the item chooser?
To draw graphics on the screen, the Game Boy Advance first has to load the graphic data into its video RAM. Since the GBA doesn't have much RAM, not everything can be loaded at once. To handle this, Nintendo divided everything into sets. Some sets can not be loaded at the same time. There isn't much that can be done about this limitation, even Nintendo had to work around it. This is one reason why in official Nintendo levels, you never see certain enemies in some types of levels.
To help you out a little, you can view an entity's compatibility page. You can click to it from within the item chooser. It will show you what that entity is compatible with and not compatible with. For example, here is Goomba's compatibility