[Performance] Mid-late game slowdown

Hi there!

So I'm using the new patch, and it may be my 'puter or the fact that I am playing in windowed mode with Firefox open elsewhere, but there seems to be rather more significant graphical slowdown in the new version after about turn 50-60 with much of the map explored:

Debug.err here

DxDiag here

Unrelatedly (I think, but not sure so I'm mentioning it) I turned up the mipmaps level and increased the graphics resolution to see if the graphics bug fix worked, and it did! Thanks devs!