I will tell you all why and I will clear some misconceptions.
"Creating and destroying ds_grids can be slow."
They are only created once in the create event. And they are only destroyed at Game End. The creation of them are not the source of the lag, however the modification of them (ds_grid_set()) may very well be.
"use vertex_create_buffer_ext()"
I am using Game Maker 8. Game Maker 8 does not have those functions, but it is a noble suggestion.
"He is talking about the chunks"
Misty is a girl, not a boy, although technically, a tomboy. Tomboy has the word boy in the name, likely causing confusion.
"not necessarily rendering"
In my past days, if I can recall I did vertex buffering before. What Binsk meant is to replace the d3d_model_vertex calls with vertex buffering functions. From what it seems, Binsk understood that it wasn't an issue of the rendering hitting the performance, but the model definitions themselves.
'Why are you using 2 loops"
Quite simply, because it is the easiest and most clear way. First I define the values of the grid, then I close the loop. Then I start a new loop, retrieve the values of the grid and plug them into the model. It is the clearest way to do it, if I did it all in one loop and defined the grid and the mesh at the same time it would cause problems with the vertex smoothing.
"why are you adding stuff in the draw event?"
Due to some strange bug in GM, defining models in the STEP event causes them to be invisible (but only sometimes.) Thus, I define them every frame in the Draw event. I must also be clear - I have not drawn them in the STEP event,simply define their vertices in the step event. And when I do so, when I draw them in the draw event they are invisible.
"You should do it all all on create"
That would be a mistake, because it would freeze and potentially crash the game. It is essential to divy up the workload into frames.
"You should do it in a timer"
I am not sure what the timer function is in Game Maker, I have never heard of it before, are you referring to alarms or timelines?