I didn't tell the whole story... I was actually fooling around with the tileset art choices in the mapscripts when I got this result. When I saved the map and uploaded it in World Builder, it showed this was the art style:
What is white here should have been a colour, representing either the American, Asian, African or European tileset. But the different tilesets must have gotten mixed up.
This obviously will make it a
lot harder to find the right blend between tiles. I'm pretty sure the two marsh tiles in my picture were of a different art style than the surrounding tiles...
Then it's not a computer specs issue. I found that other thread you were mentioning, The_J, and yeah, that indeed looks like systems not being able to handle the game.
But there are also isolated red and white blots on tiles that point to a blend problem, even with tiles of matching art sets. They mostly appear on tundra and desert in my game.
Gathered in this thread I see different type of graphical imperfections, some of them have nothing to do with this blend problem, others do. In that respect it's a bit a of a mess, this thread.