Optional.toString() is unnecessarily expensive

This is Optional.toString() https://github.com/openjdk/jdk/blob/0699220830a457959b784b35af125b70f43fa3b0/src/java.base/share/classes/java/util/Optional.java#L454

    /**
     * Returns a non-empty string representation of this {@code Optional}
     * suitable for debugging. The exact presentation format is unspecified and
     * may vary between implementations and versions.
     *
     * @implSpec
     * If a value is present the result must include its string representation
     * in the result. Empty and present {@code Optional}s must be unambiguously
     * differentiable.
     *
     * @return the string representation of this instance
     */
    @Override
    public String toString() {
        return value != null
            ? String.format("Optional[%s]", value)
            : "Optional.empty";
    }

String.format is surprisingly expensive, and we are seeing Optional.toString() showing up as a measurable fraction of CPU time in Google's servers. This code could just as easily use string concatenation, which would certainly be cheaper.

Similar remarks apply to the other Optional* classes like OptionalInt, though they are used much less.