Throttling (arbitrary frequency limitation of the CPU) happens when a sensor is reading bad, a bad battery is used or battery is absent. The machine becomes slow but not always totally unusable. Youtube video playback can become choppy at higher resolutions, moving cursor halts video, etc... The CPU temperature won't go up in most cases (and the fan will stay quite low).
- To find missing sensors, use ASD and decode here : sensors codes or use AHT/AD, not as specific.)
The following example is a 820-00165 (MBA 13" 2017 i7) with prime95 torture test starting in the middle of the graph.
820-00165 throttling due to bad battery (same with no battery). Notice the constant frequency stuck just above 1GHz. Fan stays low and temp won't go past 65°C.
820-00165 throttling due to high CPU usage (Prime95 torture test). The start of the graph is normal behavior when using programs. This time the frequency stays quite high and reach a plateau when the cooling capacity is overwhelmed. The fan is at max rpm or close.