bottleneck vs stumbling block

bottleneck

• The part of a process that is too slow or cumbersome. 

• In traffic, any narrowing of the road, especially resulting in a delay. 

• The narrow portion that forms the pouring spout of a bottle; the neck of a bottle. 

• To slow by causing a bottleneck. 

• To form a bottleneck. 

stumbling block

• A hindrance, obstacle or impediment; an opportunity for error.