I'm trying to make a Gravity Simulation for an upcoming game. It's been going smoothly so far, exempting the trajectory prediction, which is so slow I had to bind it to a key (m).

I know that I need to decrease the resolution, but I don't know how.

You replaced the proper expressions of gravity force and acceleration with random constants that give you a pleasant visual effect.

$$ (force) F = G \frac{m M}{R^2}$$
$$ (acceleration) a= \frac{F}{m} \implies a= G \frac{M}{R^2}$$ where G*M = const
$$ \Delta v = a \Delta t $$

The lower the $$\Delta t$$ the bigger the resolution (precision) of the simulation.
But simulation is played at 60 steps per second, so the perceived slowness.
With a few steps of simulation warped together, the animation looks faster without losing the precision.