I've got an AMD Dual Core 4600+, 3GB ram, Ati X1900GT video card and I've reached cities with about 7500-7800 people and seen fires burst out everywhere even on normal speed.
While Im not debating that running the game on highspeed can cause this, but it also as mentioned over at TM depends on how many prefects you have placed on the map.
When one of my cities burst out in flames, I ended up adding about 20 or 30 extra prefects to help fight the blaze, which apparently only made the matter worse since now the CPU had more calculations that it had to do for the extra prefects that I placed. However at the time that this occured my computer was still using the AMD 3500+ instead of the 64X2 4600+ that is in it now.
The way that this works makes it look pretty impossible to create really large cities with the way that the game uses the CPU cycles for the prefects intelligence.
But I guess it all depends on what you consider to be a large city. For me 15,000 inhabitants is not nearly a large city, and perhaps I would consider a large city to be more around 200,000 inhabitants which I think would bring any computer that is available today to it's knees.
So..... Not only does it concern what CPU you have, but also how many prefects you have on your map. So if your running at normal speed and have a huge map with over 50 prefects, the city may still go up in flames because the CPU just can't make enough calculations in time to combat the flames.
[This message has been edited by City Builders (edited 10-21-2006 @ 09:44 PM).]