This is stating that if you select a particular route rather than allowing TD Ameritrade’s smart routing algorithm to decide the fastest route for execution, that you may end up selecting a slower route than the best route currently available.
Therefore they are informing you that the route you have chosen might not be the best out of all of the routes and might take longer to execute than all of the available routes.
Going direct to the market maker is the fastest, but that is assuming that you have selected the best market maker at that time you placed your order.
Now the question I also have now is, is there a difference between using TD Ameritrade's algorithm and having direct access?
I have a feeling that is yes, but I don't know for sure.