Induction period

An induction period in chemical kinetics is an initial slow stage of a chemical reaction; after the induction period, the reaction accelerates.

[1] Ignoring induction periods can lead to runaway reactions.

Time is required for this transformation, hence the induction period.

The reactions to form Grignard reagents are notorious for having induction periods.

Secondly, Grignard reactions, while exothermic, are typically conducted at low temperature for better selectivity.

A sigmoid curve of an autocatalytic reaction. When t = 0 to 50, the rate of reaction is low. Thereafter, the reaction accelerates, until almost all reactants have been consumed. At that point, the reaction rate tapers off.
Wilkinson's catalyst requires activation before it can participate in the catalytic cycle