The company Analog Devices, Inc. manufactures temperature sensing devices. Their webpage headed
ADC Requirements for RTC Temperature Measurement Systems
[Analog to digital converter requirements for real time control temperature measurement systems]
contains this section
For temperature systems, the measurements are mainly low speed (up to 100 samples per seconds typically).
So from here it is clear that 40 Hz would be considered as a slow, not fast, rate of temperature sensing.
However, the marketing pitch is slightly disingenuous. The full claim is
Intelligent heat control helps protect your shine
Unlike some others, the Dyson Supersonic™ hair dryer measures air temperature over 40 times a second, and regulates the heat. This prevents extreme heat damage, to help protect your hair's shine.
The causes of hair damage have been investigated. The U.S. National Institutes of Health has this publication
Hair Shaft Damage from Heat and Drying Time of Hair Dryer
The study assessed changes in the ultra-structure, morphology, moisture content, and color of hair after repeated shampooing and drying with a hair dryer at a range of temperatures.
. . .
Diverse causes of extrinsic hair shaft damage have been documented, and can be roughly divided into physical causes and chemical causes. Chemical causes include bleaching, hair dyeing and perming. Frequent use of chemical agents is a major cause of damage to the hair shaft. When cosmetic products are used incorrectly or too frequently, they may produce changes in hair texture that correspond to morphological changes on the hair surface. Physical causes of hair shaft damage include friction from hair accessories, washing, and towel drying. Friction is a major damage factor of the hair surface, especially in wet hair, although other factors, such as photodamage and daily grooming may also lead to hair damage. Exposure to ultraviolet radiation damages hair fibers and sunlight can lead to dryness, rough surface texture, decreased color and luster, and increased stiffness and brittleness.
But tellingly it also says
Hair dryers, which are commonly used for drying hair, also can cause hair damage. The patterns of heat damage caused by hair dryers have been investigated. Yet, the best way to dry hair without damage remains unclear.
So while regulating the temperature may help with hair condition, Dyson's claim that
This prevents extreme heat damage, to help protect your hair's shine.
seems to be 'marketing speak'. A standard way to prevent "extreme heat damage" is with a mechanical bi-metallic strip switch, and it may be that the device also contains one, for safety reasons.