Benchmarking String.toInt() vs Checking individual characters gives unexpected result
As pointed out by Majenko in his (slightly annoyed) comment, you need to change your timing code to time only the calculations you want benchmark, not the I/O or other irrelevant code. For checking the conversion using toInt(), for example, try:

// time our conversion
unsigned long start = micros();
unsigned long rInt = r.toInt();
bool invalid = rInt == 0;
unsigned long end = micros();

// print the result
if (invalid) {
    Serial.print("in");
}
Serial.print("valid conversion took ");
Serial.print(end - start);
Serial.print(" microseconds.");


Note that your assumption that a three-character input that's a valid integer will be between 100 and 999 is wrong; "003" and "-17" are both three-character inputs that should be correctly converted.

According to the toInt() documentation, 0 is returned on failure to convert, so I check for that in the code above. But since one can't tell the difference between a successful conversion of "0" and unsuccessful conversion of "abc" or some other non-integer, you really should be sure before you call toInt() that you've got a valid integer int the string.