Seth has a great explanation of Optimal String Concatenation in UserTalk

UserTalk scripts often need to build large strings from smaller ones, such as when assembling a web page or RSS feed in Conversant, Manila, or even in the old static site engine. In order to optimize this process as much as possible, there are two things the scripter needs to remember: "Avoid copying large strings whenever possible," and "trigger the secret in-place append." [more]...