Since many lag problems are caused by a speck of routable territory in the middle of unroutable land, it can be good to start with an unroutable flat map - at least near the edges - and erase the unroutable paint  just ahed of placing roads and lots.  Use "show routing" and "rebuild routing" regularly to ensure all your routing spots are connected, with no unconnected dots.

Contrary to what you may see in routing tutorials, "broken" routing lines do not mean there are routing issues - the line is simply being rendered underground.  With a few exeptions for glitches, your sim will NOT sink into the ground when running over routable hills if routhing has been rebuilt before being exported for play.