Key performance for a dictionary

No. First of all, Dictionary [UPDATED] uses hash code of the keys to find them in its internal storage - rather than the keys. And Hashcode is an int. For int, it is just the value of the int, for string it has to be generated.

So using int is slightly faster.

In fact generating hash code for a string is a pretty complex process (snippet using Reflector) [Hope this is not taken as copyright breach because it is NOT]:

fixed (char* str = ((char*) this))
{
    char* chPtr = str;
    int num = 0x15051505;
    int num2 = num;
    int* numPtr = (int*) chPtr;
    for (int i = this.Length; i > 0; i -= 4)
    {
        num = (((num << 5) + num) + (num >> 0x1b)) ^ numPtr[0];
        if (i <= 2)
        {
            break;
        }
        num2 = (((num2 << 5) + num2) + (num2 >> 0x1b)) ^ numPtr[1];
        numPtr += 2;
    }
    return (num + (num2 * 0x5d588b65));
}