the.com/slow burn

means A gradual build of attraction or tension that develops over time rather than erupting all at once.

from Burn has long described smoldering emotion — anger or desire that builds rather than flares. 'Slow burn' as a phrase took off in 20th-century entertainment writing and screen acting, describing a character's mounting frustration or feeling that simmers before it finally shows on their face. From there it spread into storytelling criticism generally, including romance, where it now names a relationship arc that resists instant sparks in favor of a long, patient fuse.