Currently, for all civilizations, trades take 25 seconds to create. The Mongol landmark gives a +50% production speed bonus, but trades take 16 seconds instead of 12.5 seconds.
16 seconds is correct.
Original production speed: (1 unit)/(25 seconds)
1/25 = .04
.04 x 1.5 = .06 = 1/16
New production speed: (1 unit)/(16 seconds)
You are confusing production speed with production time.