A slow site puts users off — many will leave after a few seconds if nothing loads. That means higher bounce rate and fewer conversions (enquiries, sales).
Google considers speed when ranking (Core Web Vitals). Faster, smoother sites can rank higher. Optimization includes: images (format, size), code (CSS, JS), hosting, caching.
At launch it’s worth using optimized images, sensible script loading and good hosting. You can then measure speed (e.g. PageSpeed Insights) and improve weak spots over time.