Can a Calorimeter Measure Fast Chemical Reactions?

[Sonim]

TL;DR

Question about enthalpy and fast reactions.

My book introduces Thermochemistry with the concept of heat and how a calorimeter works. After that, it explains the story behind Hess Law and says that one of the reasons it was created is because a calorimeter can't calculate fast chemical reactions, but it doesn't really say the problem with it. Why can't it calculate fast chemical reactions? Won't the water in the calorimeter heat up or cool down after some time?

 

[Borek]

How are you going to determine a heat of a single step of a quick multistep reaction?

 

[Sonim]

Supposing a fast single step reaction, is it possible to calculate its enthalpy variation using a calorimeter or I would need Hess Law to calculate it using other reactions?