There are many tools out there now to monitor your website performance and to help you flag up those bottlenecks. The first approach is to find out your score using a couple of tools. I tend to use Google’s own pagespeed
tool which utilises the Lighthouse Lab as it’s basis and GTMetrix
which combines YSlow and Pagespeed together.
Pay particular attention on red values and use the Waterfall to see which files take the time to load. If it’s an image try to optimise it by cropping the size, using a different format or managing the image quality. If it’s a css or js file, make sure they are minified and maybe combine into one. If using WordPress you can use Autoptimize for example.