Mobile Performance Fix for Site

My 10–12-page website feels noticeably sluggish on phones and tablets. I need you to diagnose what is holding back the loading speed and deliver a single, clean code adjustment or snippet that can be applied site-wide to bring every page up to modern mobile-performance standards... (Budget: ₹600 - ₹1500 INR, Jobs: CSS, Frontend Development, HTML, HTML5, JavaScript, Mobile Development, Web Development, Website Optimization)