A website can look great, give great information, and be feature rich but it will all be for nothing if performance is poor for the user. Performance can be affected by a number of factors, including network speed, non-minified resources, image sizes, compressed files, font file size and the number of requests made to the server.
There are some web tools that allow you to test a website for performance and these give a wealth of information that can be acted upon.
It may not be possible or even necessary to act upon every suggestion if performance is already good but it's still work a look at these sites.
