First, tires are always sold as individuals with the rare (and expensive) exception of front/rear specific designs.

Second FBinNY is quite correct that a narrower tire isn't always faster. What you might notice is a weight difference. 700-28 tires are usually intended for touring or utilitarian use and can be quite heavy to improve durability. A "high performance" 700-23 tire will often be much lighter ( and less durable) and the weight difference can easily exceed 100 grams per tire.