For those worrying about trade units slowing the game down remember what calculations cause the AI to take a lot of time.
All trade units exist on their own strategic layer and ignore both military and civilian units. The main slow down for the AI is the same one you have; navigating the mess that is moving a lot of units in the proper sequence to avoid traffic jams in 1upt. Trade units do not care. Since it almost certain there is no stacking limit for trade units on the map, what possible benefit would this provide, they will need no calculations to avoid traffic or be worried about interrupted movement.
Secondly, the trade units are on a fixed and predetermined path. The AI takes a lot of time crunching its options for unit movement. For trade units there is only one option. There is no calculation about who to attack or what improvement to build, or what city to spread religion too etc. Add these together and I think , and hope, that trade units will not be the drain on turn times people fear.