A longer stem will slow the steering down, Whether that is good or bad depends on how quick the bike steers and what level of quick steering you want or are comfortable with.

Far more important - the stem plays a big role in your comfort on the bike, where and how you sit on the seat and your position in regard to aerodynamics. On the comfort side - back bend. Too long and you may suffer reaching too far and/or bending over to much. Too short and you may find your back cramped. I find my breathing and hence almost everything else is better when I have a stem that is long enough.

Your body type has a lot to do with what stem works best for you. Your riding style also. Something I have observed (and is really easy to use for experimentation) - the correlation between stem height and stem length. For me, once I find a stem length and height that works for me, I can change to a longer or shorter stem and adjust the height and get near identical riding comfort and efficiency. Again, for me, I can shorten a horizontal stem by 2 cms and push it down one (or remove one cm spacer on a threadless stem) and there's very little change. So, working backward, I can raise or lower the stem to experiment and see if longer or shorter is better.

Your stem is just about mid-height judging by the stem I see showing. Try dropping it a cm. Ride that for 3 weeks. Better? Longer just might be better for you. Worse? Well now you know and it cost you nothing but 10 minutes of your time.