Looking at the performance meter in the top left, you can see how much RAM it's using. On Chrome (which I use) it's really how much RAM all the Flash processes in all open tabs are using, which can be a lot. When the number gets big and things get slow, I kill the Flash plugin by clicking the Chrome menu, then More Tools, then Task Manager, then sort the tasks by memory, then kill off "Shockwave Flash". All the stupid ads that were sucking up your memory will be dead, and you can reload Grid12 and go fast.
Another problem is big maps. The map takes up a lot of memory and big maps make things slow for everyone. Sorry about that.