What is the likely cause if a car is very sluggish pulling away from a stop but performs better at highway speeds?

The situation described, where a car feels sluggish when pulling away from a stop but performs better at highway speeds, can often be attributed to issues with the torque converter. The torque converter is a critical component of an automatic transmission that helps to multiply engine torque and allows for a smooth transition from a complete stop to moving speed.

When the vehicle is sluggish at lower speeds, this indicates that the engine may not be effectively transferring power to the wheels. A defective torque converter may not be locking up properly or may be slipping excessively, which can lead to poor acceleration and performance during initial takeoff. However, once the vehicle reaches higher speeds, the torque converter's operation can be less noticeable, and the car may perform more satisfactorily.

In contrast, the other options present different issues that would likely have other symptoms. For example, high main line pressure could lead to harsh shifting and potential engagement issues, low governor pressure could affect speed-sensitive shifting, and a plugged transmission filter would typically manifest in overall performance loss, not just sluggishness at low speeds. Therefore, the symptoms described align closely with potential torque converter failure.