As far as resolution goes, for this shot you could have easily choosen an wider Aspect ratio Image, as about the lower half of the Image as of now is "dead Space", where almost nothing is happenig anyways. So if you went with half the vertical resolution and reframed the shot to only see the now top part where the pellet is flying, you wouldnt really loose any detail from that, while increasing framerate by quite a bit. For Tracking things like those in Flight, a pretty Wide image is usually more usefull anyways than an close to Square one.
.
Another Trick to make the Projectile seem to move slower, i see often used with projectile Footage is, to not shoot at an 90° Angle from the Path of flight (Of course sometimes necessary to get an good view of front and back of target...), but to get closer to the Shooters Perspective or an more shallow angle, as now the same Space in Frame covers more real World Distance the Pellet will have to travel, meaning it will be in Frame for longer (and also moves seemingly slower).
Of course not very usefull trick, if what you are going for is the typical 90° Side angle, but still worth thinking about sometimes.