From seconds to years

"Fast" and "slow" are not just two boxes. The time a reaction takes is a continuous range, running from a fraction of a second to a matter of years.

In a second — a firecracker going off
In a minute — zinc dissolving in a dilute acid
In an hour — a meal being digested in the stomach
In a few days — a mango ripening
In a few months — a sheet of iron weakening with rust
In several years — an iron grille on a building rusting away

Some reactions, then, are instantaneous or very nearly so, while others go extremely slowly. The difference between them is not whether the reaction happens but how long it takes to happen.

This matters in practice. Food that spoils quickly cannot be eaten; cement that sets too quickly cannot be laid. What we need next is a precise definition of this "speed".