Breakneck and Post-haste

Breakneck adjective - Moving, proceeding, or acting with great speed.

Post-haste and breakneck are semantically related In some cases you can replace phrase "Post-haste" with "Breakneck", this terms are similar.

Post-haste adverb - As fast as possible; with all possible haste.

Breakneck and post-haste are semantically related in fast topic. Sometimes you can use "Breakneck" instead the phrase "Post-haste" as an adjective or an adverb, if it concerns topics such as speedy.