Notion’s custom emoji feature looks small, but it forced real engineering decisions: how to represent emojis cleanly inside a rich-text editor, keep the emoji picker fast under heavy libraries, and choose a loading strategy that feels instant without blowing up startup performance. The key idea is treating custom emojis like “mentions” (stable references by ID), then using practical performance tactics like virtualization and “preload now, scale later” guardrails (like a workspace cap) to ship fast and evolve later.