What causes lag in a level?

[Kobor Nefleheim]

Greetings, Kobor here:
I was pondering recently over what causes an SMBX level to cause intense lag?
I had possessed a level which I'm know recreating known as the "Earthbound Stronghold". Everything was fine, the design; the intricate events and placement of npcs. Yet I have experienced major lag in that level? Can anybody have any suggestions or answers to this? I have recreated the level so no link is accessible to the level itself; but out of my concern for future SMBX attempts, I would like to know. Also other levels made by me and others have quite an amount of stuff within it but it runs perfectly fine and playable.
Edit: Before attempts of answering are upholded i would suggest may it be because of the amount of npcs placed together in a section?

[FallingSnow]

Excessive custom graphics (player graphics lag the game for some people).
Excessive moving layers.
Excessive sparkies or sparkies on moving layers.
Excessive sizeable blocks.
Excessive npcs or effects on screen.

Those are some of the most common.

[zioy]

Note that most of the correct responses in this thread are likely to cause crashes in the editor, but not the actual game.

[superhammerbros100]

The lag goes away when you close and re-open the editor. But after alot of time the lag comes back.

[RudeGuy]

Too big sizables too cause a lag.

[Julia Pseudo]

Sizables are definitely the most noticeable cause of lag. Make sure you don't use too many big ones on the screen at once.

[YoshiGo99]

I usually get lag when music ends and when it starts back up, well it's more when I don't use music at all, it lags. I don't know what the reason is, I always use mp3s. It might be fixed in the recent version, I still have to update mine.