bottleneck has 2 distinct synonyms spread across 1 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a narrowing that reduces the flow through a channel
the narrow part of a bottle near the top
Synonyms, antonyms and the broader and narrower relations for bottleneck come from Princeton University’s WordNet. Full sources and licences.
slow down or impede by creating an obstruction
“His laziness has bottlenecked our efforts to reform the system”
become narrow, like a bottleneck
“Right by the bridge, the road bottlenecks”