The source code of the Java API is freely available. Here's the parseInt() method. It's rather long because it has to handle a lot of exceptional and corner cases.

public static int parseInt(String s, int radix) throws NumberFormatException {
    if (s == null) {
        throw new NumberFormatException("null");
    }

    if (radix < Character.MIN_RADIX) {
        throw new NumberFormatException("radix " + radix +
            " less than Character.MIN_RADIX");
    }

    if (radix > Character.MAX_RADIX) {
        throw new NumberFormatException("radix " + radix +
            " greater than Character.MAX_RADIX");
    }

    int result = 0;
    boolean negative = false;
    int i = 0, max = s.length();
    int limit;
    int multmin;
    int digit;

    if (max > 0) {
        if (s.charAt(0) == '-') {
            negative = true;
            limit = Integer.MIN_VALUE;
            i++;
        } else {
            limit = -Integer.MAX_VALUE;
        }
        multmin = limit / radix;
        if (i < max) {
            digit = Character.digit(s.charAt(i++), radix);
            if (digit < 0) {
                throw NumberFormatException.forInputString(s);
            } else {
                result = -digit;
            }
        }
        while (i < max) {
            // Accumulating negatively avoids surprises near MAX_VALUE
            digit = Character.digit(s.charAt(i++), radix);
            if (digit < 0) {
                throw NumberFormatException.forInputString(s);
            }
            if (result < multmin) {
                throw NumberFormatException.forInputString(s);
            }
            result *= radix;
            if (result < limit + digit) {
                throw NumberFormatException.forInputString(s);
            }
            result -= digit;
        }
    } else {
        throw NumberFormatException.forInputString(s);
    }
    if (negative) {
        if (i > 1) {
            return result;
        } else { /* Only got "-" */
            throw NumberFormatException.forInputString(s);
        }
    } else {
        return -result;
    }
}

I invite you to look at the source code of the parseInt function. Your naïve function is faster, but it also does way less. Edge cases, such as the number being to large to fit into an int or not even being a number at all, will produce undetectable bogus results with your simple function.

It is better to just trust that the language designers have done their job and have produced a reasonably fast implementation of parsing integers. Real optimization is probably elsewhere.