Why is the ester hydrolysis slow in the beginning and becomes faster after sometime?

The hydrolysis of an ester :
RCOOR' + H₂O ⇌ RCOOH + R'OH

One of the products is carboxylic acid, which dissociates in water to form hydrogen ions. When the reaction goes on, the concentration of carboxylic acid is increasing, leading to increasing concentration of hydrogen ions. Hydrogen ions act as the catalyst of the hydrolysis, so that the reaction becomes faster and faster due to the increasing concentration of hydrogen ions.