A 5x5 is a very large embark. If you have an embark that big on top of a large world map with a long history then I can see it causing serious performance problems for even a very high end computer. As other have said, the fact that the game is becoming unresponsive sounds like this may not be just normal FPS loss. You should definitely see how the game responds when you scale everything back a bit.
A 5x5 is a little under 3 times the size of the standard 3x3 embark and more than 6 times the size of a 2x2. If you want to capture multiple biomes and geographical features I suggest creating long thin embark area instead of a square one, something like a 5x2 or even a 2x6. Both should run much better than a 5x5 and still give you a lot of space and biome/terrain variety to work with. I personally tend to aim for the product of the two dimensions equaling 12 or less.