Test String.Concat() StringBuilder
1000x10 char strings 4 ms 9 ms
1000x50 char strings 20 ms 10 ms
1000x500 char strings 1852 ms 100 ms

Table 1
Time is of the Essence. Using String.Builder for a few Append operations often is sluggish compared to String.Concat(), but it's faster for strings created in loops. Here are the times for String.Concat() and StringBuilder.