'slow-building' originates from English, specifically a compound of 'slow' and the present participle 'building' (from 'build').

'slow' originates from Old English 'slaw' meaning 'not quick, sluggish', and 'build' originates from Old English 'byldan' (related to constructing); the compound 'slow-building' is a modern English formation combining the adjective 'slow' with the participle 'building' to describe something that grows or intensifies over time.

Initially the elements meant 'not quick' ('slow') and 'to construct' ('build'); together as the compound they evolved into the descriptive sense 'growing or intensifying gradually' used in modern English.