Route finding is one of the most memory hungry actions the AI can undertake. Yes it is a balancing act as to whether you can spare the cost of not having sea trade but it is deffinately a good choice.
You get a similar thing with some other games, for example real time stratagy games can be set up to feature friendly fire, but the speed reduction means it almost never finds its way in to comercial releases.
I'd suggest working the modification in to the mod from the very start.
Also make units very expensive, so that there are fewer of them (less comp resources needed) and have more expensive settlers. Also think about removing railroads. You can replace them with roads to represent the fact that before the rail road resources were often only developed in to goods localy. This really depends on your scenario, but its another thing that will speed up turns greatly.
One thing to take in to acount, with small numbers of cities the player will have an advantage over the AI, as the AI usualy uses it's ability to quickly manage large numbers of troops and cities to beat the player's ability to think stratagicly using just a few units. So get the player to use a higher difficulty, or give the AI civs an advantage.
