Determining if a C-style string is alphabetic
The main problem with your code (in terms of performance) is that it processes every character in the string regardless. It's probably better to actually exit early if you find a non-alpha character, with something like:

bool isStrAlpha (const char *chPtr) {
    // if (*chPtr == '\0') return false;
    while (*chPtr != '\0')
        if (! isalpha (*chPtr++))
            return false;
    return true;
}


And just a couple of extra notes:


This code works fine even for zero-length strings, assuming the intent is to ensure whatever characters are in the string are all alpha. In other words, it considers an empty string valid because it contains no non-alpha characters. If you want to ensure the string has to have at least one alpha, just reinstate the commented-out line at the start of the function.
Your second assumption is superfluous. A series of characters that are not terminated with \0 is not actually a string in C.
Whether alpha characters are likely to be consecutive is not relevant in my opinion, I usually prefer correct, portable code over speed. But, in any case, you can be reasonably certain that the people who created your implementation will have optimised the wazoo out of the character classification stuff.