Recently the queue for processing cells changed. Before it was a simple nested pair of for loops that went through every cell in a rectangular region slightly larger than the screen. After it uses a substantially more complicated system, where the changed cells from the previous frame are added into a list, the list is sorted by the distance to the player's location, and the first few hundred closest cells are handled, with potential changes from those cells added back into the main list.
This is good for some things, and bad for others. It makes the preliminary world simulation (two of the generation steps) much faster, and means, if there's nothing much happening near the player, we automatically simulate progressively further areas of the map.
However there is a problem, and the more experienced programmers probably already see what it is. It's that sort. Before the change it didn't matter much how large the world was; if it was 2x2 or 100x100, it took the same time to calculate. The sort operates on every change-flagged space on the map, and it makes a function call for every flagged cell on every frame. That's roughly 7,000 cell coordinates in a newly-generated terrain world, every frame.
You can actually see for yourself, a bit, how much time the sort takes up. When you start playing in the prototype, if you turn on the debugging HUD you'll be told the framerate. On my low-end, single-core laptop, I generally get framerates in the mid-to-high 20s. The simulation tends to find an equilibrium over time though, and as water pools fewer nearby cells are calculated each frame, meaning the range that gets calculated expands, ultimately decreasing the size of that sort list. The result is, over time, framerates slowly increase.
This is my primary development concern at the moment. I'd like to increase framerates by a bit more so that I have more processor time to do effects (maybe related to making fluids less blocky), increase the complexity of the simulation, and perhaps draw a background layer. Right now I'm getting a handle on Python's profiling tools to make sure my suspicion is correct, but even if it turns out not to be THAT bad I can't help but think I could design this better. Maybe going back to the rectangle, but increasing its size as visible activity decreases.