Well yes, working with strings in terms of doing a large number of operations on them will be slow, that's why in dot net we've got the speed demon StringBuilder Class that is almost oddly faster that fast. Use that guy if you're gonna do tons of string operations. If you're just gonna perform a one or just a few operations on a string then just a plane ol string method is good enough, you won't notice the performance hit until the number of operations gets up there.
VB Code:
  1. Dim str as string

  2. str = "hello"

  3. str = "world"
The above code is a little different than the other examples. This code says on the second line, assign the value of "hello" to my string variable str. The third line says, assign the value of "world" to my variable str. What's important to note is that the space in memory that holds "hello" is not modified to hold "world", what happens is that a new space is created to hold "world" and that space assigned to the variable str.