What can cause poor performance in the HVAC blower motor?

Poor performance in the HVAC blower motor can indeed be attributed to both a faulty speed switch and a disconnected wiring harness. When the speed switch malfunctions, it may not effectively control the blower motor's operation, leading to inadequate airflow or the motor not functioning at all. The speed switch is responsible for regulating the different fan speeds, so a fault here will directly impact performance.

Similarly, if there is a disconnected wiring harness, power may not flow to the blower motor properly, which can cause the motor to operate intermittently or not at all. Wiring issues can create breaks in the circuit, preventing the electrical signals needed for the blower motor operation from reaching it.

While a missing cabin air filter can impact airflow quality and efficiency, it typically does not directly cause poor performance in the actual motor operation itself. Therefore, the combination of problems presented in both the faulty speed switch and the disconnected wiring harness is what leads to the correct answer, which encompasses both potential issues impacting the HVAC blower motor's performance.