The coolant "circuit" through the heater core with the FB is different from the previous engine configurations, and this will lead to a later appearance of warm air from the heater. Ironically, the reason for this intentional delay is to speed up engine warming, as usual, for fuel and emission purposes.
With the previous engines, the heater core is connected to the engine so that coolant is always flowing through the core -- one connection is at the top of the engine, at the water (crossover) pipe, and the other is at the inlet to the water pump. The flow is from the pump, through the engine (around the cylinders and heads) up to the water pipe, to the heater core, and through it back to the water pump inlet.
When the thermostat opens, coolant will also flow (in parallel) from the water pipe to the top of the radiator, down through (or across) the radiator, out the lower radiator hose to the thermostat and then to the pump inlet.
With the FB, the coolant flow through the heater circuit is reduced when the thermostat is closed, and only opens when the thermostat opens. This is effected by a two-sided thermostat. (See attached picture.) The amount of heat available for heating the cabin is reduced until the engine is fully warmed up. But, because engine heat is not being lost to the cabin, the engine will tend to warm up faster than the earlier models.
The apparent absence of cabin heater output in the early stages after cold start up isn't because the engine is slow to warm up.