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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 either stay quite low or ramp up fully).

Intel Power Gadget can be used to monitor the power consumption and the actual core clock of the CPU to observe proper boosting behaviour under load. Intel Power Gadget is only available starting from 2nd gen Core CPU. Trying to run it on 1st gen Core will result in an immediate kernel panic. Instead, while it is much less user friendly, the following command can be used in the Terminal to monitor the actual core clock, refreshing every 0.1s:

sudo powermetrics --sampler cpu_power -i 100

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 gradually ramping up at max rpm or close.This is the expected behavior.

Case study : 820-00165 (i5 early 2015) throttling eratically under load, system very slow. Notice the frequency capped around 1GHz, the low power used and subsequent low thermals (under 65°C).
Case study : 820-00165 (i5 early 2015) After the fix. A bad cap on PP3V3_S5_AVREF_SMC was messing with the SMC itself. System behaves normally under load. The frequency now scales up and stays there (normal thermal throttling) as temps rise up to 100°C (Prime95 small FFT is torturing the poor Air).