2. While You Were Sleeping FL falls for ML as he rescues her from the accident she can't evade - and the slo-mo and BG music are right there to drive the point home, LOL.
3. The Untamed Lan WangJi slo-mo'es his way into Wei WuXian's heart on that path at the entrance to Cloud Recesses, even if it takes WWX the rest of the story (well, the whole novel, actually LOL, because damn drama censors *sigh*) to realize it.
I propose that Gu Family Book actually has two such instances - first for Wol-Ryung and SeoHwa (their meeting in the forest, with her committing suicide in slo-mo and him preventing her from doing it) and then KangChi and YeoWool under that pear tree in the moonlight that is supposed to show their love is doomed (though I don't remember if that second instance is slo-mo too, LOL). So I won't number it in case it doesn't fit the theme after all.