How to keep dark mode fast
Table of contents Why performance matters for dark mode Common performance pitfalls Critical CSS strategy Fast theme detection and FOIT prevention Minimizing assets and safe swapping Efficient JavaScript patterns Image and media optimizations Testing and measuring performance Quick production checklist FAQ Why performance matters for dark mode Dark mode should feel instant and unobtrusive. If toggling theme adds measurable delay, causes layout shifts , or increases page load time, users perceive the site as sluggish and unreliable. For Mediumish — a content-first theme — readers expect fast rendering and smooth interactions. A slow dark mode can lead to higher bounce rates, poorer user engagement, and lower perceived quality. Performance matters not only for UX but also for SEO: slower pages often score lower on Core Web Vitals and search rankings. The goal is to add dark mode while keeping initial paint , interaction re...