The first time a web browser loads a page of your site, the external css file will be loaded into memory along with the rest of the page. Hereafter, however, this style information (as long as it doesn't change) will be cached; reducing page load times.
Of course, this means that you have to have somewhere to host the external css file. Somewhere reliable (if this site goes down, but Blogger remains OK, your site will look a bit less than its usual self). If you have somewhere, however, it's highly recommended to make the most of it.
To link to the external stylesheet, simply place the following line anywhere in the head section of your Blogger template :
<link rel="stylesheet" type="text/css" name="anyname" href="url">
