Hi! I am not 100% sure what you are wanting, but I have a couple of things to try. First you could change the time scale of a tween by using TweenSet()...
TweenSet(tweenID, "time_scale", 0.5);
You could also change the time scale for the whole tweening system like such...
TweenSystemSet("time_scale", 0.5);
Lastly, you can change the update interval of of the tweening system...
TweenSystemSet("update_interval", 2);
The default interval is 1 which updates the system every step. But if you set it to 2, it would update every 2 steps. This means that if your game ran at 60fps, then the tweening engine would run at roughly 30fps.
I hope this helps. Follow up and let me know if any of this works for your needs :)
-Stephen