Slow speed reaction:
The reaction which requires long time for completion i.e. occurs slowly is called slow speed reaction.
Examples:
(1) On heating potassium chlorate (KClO3) it decomposes slowly into potassium chloride and oxygen gas.

This reaction requires long time for completion, therefore it is slow speed reaction.
(2) Rusting of iron is a slow speed reaction. In this reaction iron reacts with oxygen from air to form iron oxide.

This reaction requires long time for completion.
Fast speed reaction:
The reaction which is completed in short time i.e., occurs rapidly is called fast speed reaction.
Examples:
(1) The reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl) is a neutralization reaction and it is fast speed reaction.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
This neutralizaton reaction is completed in short time, therefore it is fast speed reaction.
(2) Aqueous solution of sodium chloride reacts with silver nitrate solution to form white precipitate of silver chloride (NaCl) and sodium nitrate.

This reaction is completed in short time, therefore it is fast reaction.