Alan

I believe it does actually get hottest at the slowest speed, the current is lower but the voltage drop is at its highest and the self cooling airflow is also at its lowest. Having the debris in there further reduces resistor cooling and also makes for a stink when the resistor element gets hot. Degraded bearings and/or major airflow obstructions will have most effect at low speeds (e.g. at 0 speed which is Not off). The fan may not operate at all or run significantly slower than normal, this is the classic case for magic blower syndrome (slowest -> fastest speed).

Alan