How do I make a 'for every tile' with a tilemap object?

I know that it is possible to do this with arrays, but that just makes my game EXTREMELY slow(1-2 FPS) and I need a 'for every' with a tilemap object similar to 'for every item'.

1 to 2 fps means you made a few wrong assumptions about what is viable, so looping over a grid is not your biggest issue.

First of all, yes I want it to check CONSTANTLY. If that would be very resource demanding, what else can I do? I am trying to make it so if ANY tile on the tilemap is a certain tile, it spreads to blank tiles.

You haven't provided any information other than poor performance.

Best guess would be literally the biggest thing in the game.

If that would be very resource demanding, what else can I do?

In general, I'd say, go to the events that CAUSE any tile to change to that specific state and insert the desired action there instead of checking the ENTIRE tilemap constantly. If that's possible - tilemaps are a bit of a specific thing in Construct 2.

If you want, maybe you can describe what exactly you are trying to achieve?

This is what I have so far (_https://www.dropbox.com/s/uhb898mzbu8tg3n/BuildingGame.capx?dl=0_). I am trying to make it so if ANY tile in the OBJECTS tilemap is the GRASS BLOCK tile and the tile above is the DIRT or GRASS BLOCK tile, it becomes a DIRT tile. Similar to the image below.