You can find setup instructions here: [google.com...]
Be aware that enabling Site Speed tracking for your site results in an extra request made to the Google Analytics servers, separate from the page tracking GIF request made for pageviews. To minimize impact on overall site latency, this request is sent only on a sampling of pageviews to your site.
Whether an average page load time of 5 seconds is acceptable depends on how much work you've put into optimizing so far. In other words, is 5 seconds the best you can do? Use the Site Speed panel to discover the worst-performing pages.