A low ram concentrator is just what it sounds like. The coordinator device does not have enough memory to feel a full routing table so it only stores a partial table. When the coordinator needs to send a message to a device but does not know the route, it first has to send out a discover frame and wait for a routing response before it can actually send the message.
A high-ram concentrator is the opposite. It maintains a full routing table to each end device. This also means that a high-ram concentrator has to frequently request route information from devices that act as routers so it keep its routing tables updated. Since the coordinator knows the routes to each device, whenever it needs to send a message it can do so without having to wait for route discovery. This method usually results in much quicker response times to devices.
