Yes, it is generally true. There is not a big dependence on flight speed because the inlet on jet engines slows down the flow before it enters the engine. So ev

Yes, it is generally true. There is not a big dependence on flight speed because the inlet on jet engines slows down the flow before it enters the engine. So even though an airliner might be flying at Mach 0.85, the face of the engine only sees about Mach 0.3. The reason the duct improves the thrust of the engine is because it reduces the pressure losses (improves efficiency). A fan (or any stage in the engine compressor) increases the pressure of the air. Since air moves from high pressure to lower pressure, the air that you just pressurized wants to sneak back around to the front of the fan around the tips. By putting a duct around the fan, you help keep air from sneaking back and robbing you of the work that you just did on it. In fact, the efficiency that you gain is highly dependent on the clearances between the duct and the tip of the fan. A lot of work goes into reducing these clearances as much as possible. You can imagine that that is a non-trivial task since the parts of the engine expand and contract based on temperature, and also vibrate and have dynamic structural behavior.