Text type event variable is equal a string or stringbuilder?

On a event sheet i making a function and that function need to modify a text type variable very often. This is a big problem for the performance and memory because string tipe is immutable tipe. I want to use a another type like stringbuilder in c# language for the solving this problem. Possible that?

From what I read most browsers already do this internally with JavaScript. Are you having performance issues?