Short answer
A slow site loses customers because people on phones give up if a page does not open within a few seconds, and Google uses speed as one of its ranking signals. The usual causes are large images, too many plugins and poor hosting, and speed can be checked with the free PageSpeed Insights tool.
Why does speed matter?
Google uses speed and page experience (Core Web Vitals) as ranking signals. According to Google’s research, more than half of mobile visits are abandoned if a page takes longer than three seconds. On phones, where most searches come from, speed matters even more. Local SEO depends on it.
How do you check speed?
Google PageSpeed Insights is a free tool. Enter the address and get a score from 0 to 100, separately for mobile and desktop. Above 90 is excellent, below 50 needs fixing. Always look at the mobile score, because Google indexes the mobile version.
What slows a website down?
Large unoptimised images, too many plugins, poor hosting, uncompressed code and third-party scripts. A redesign and maintenance include speed optimisation as a standard part of the work.