If you are worried that adding Javascript code of Google Analytics to your website will slow down the site, that’s not the case. From Google support:

The appearance of your website will never be affected by your use of Google Analytics – we don’t place any images or text on your pages. Likewise, the performance of your pages won’t be impacted, with the possible exception of the very first page-load after you have added the tracking code.

This first page view calls the JavaScript on Google’s servers, which may take slightly longer than a regular page load. Subsequent pageviews will use cached data and will not be affected.