IPA: /ɪmˈpiːdər/
KK: /ɪmˈpidər/
A person or thing that slows down or prevents progress or movement.
The construction work was an impeder of traffic flow in the city.
Impeder is formed from "in-" (meaning not) and "pedere" (meaning to impede or hinder). The word describes someone or something that causes obstruction or prevents progress.
Think of someone who is 'not' ('in-') allowing movement or progress ('pedere') — that's why an impeder is someone who hinders.