As other have already told you - one 3s battery will give you poor performance and low speed. Getting a second battery (match the exact same battery you have now), will double your speed.
If you want to figure out speeds - use an online speed calculator:
http://calc.esk8.it (pretty nice - kudos to whomever made this)
Now you can look at gearing changes to hit the speeds you want. First i’d go with more voltage - assuming your ESC can handle more than 6s. Make sure of this before you go higher voltage than 6s (22.2v nominal).
When you want to go faster you will typically adjust voltage first, then gearing. Last a higher kv motor (but that’s pretty $$$, so i’d avoid if possible).
If you really want to go fast, i’d also suggest a different skateboard deck/type. The short double kick skateboard in your video will be tough to control at higher speed (your skill obviously unknown). A longer longboard deck, or downhill deck would be easier to control at higher speeds.