Firefox is playable too but slower. IE 9.0 is unplayable unless you have a very good computer.
The next thing that makes a big impact is the size of your screen. The game area is adjusted to use your entire screen. The higher resolution the more graphics to draw and the slower it gets. If you are playing on a slow computer and/or lousy browser you can shrink the window and reload the page. This will reduce the amount of tiles to draw when you scroll around the map and move units which is the most performance consuming task.Each tile is 64 x 64 pixels and by reducing the height or width by 64 pixels you will draw one row of tiles less.