Does particle's speed affects double slit result? Just curious did anyone try to do double slit using particles traveling at the slowest speed possible, imagine the particle trajectory subject to Earth's gravity and the screen is on the floor. This may sound preposterous even by my standard but I like to know do speed matters or not, thanks.
 A: Not exactly what you seek but close: the diffraction of $C_{60}$ molecules by a grating [ANVA+99]. The speed was about 220 m/s. An interference pattern was observed.

The bottom picture shows the profile of the beam without the grating and the top one shows the outcome with the grating. The solid line is a fit using Kirchhoff diffraction theory.
So now, a bit of theory! The relevant parameter is the de Brolie wavelength of the particles, which is $\lambda=h/p$ where $h$ is the Planck constant and $p$ is the momentum. The width of the slits in the grating has to be approximately equal to $\lambda$ for diffraction to happen. For slow moving particle such as in this experiment, the Newtonian version is enough: $p=mv$ where $m$ is the mass of the particle. So yes, the speed $v$ matters but the theory accounts for it.
[ANVA+99] Markus Arndt, Olaf Nairz, Julian Vos-Andreae, Claudia Keller, Gerbrand van der Zouw, and Anton Zeilinger. Wave-particle duality of c60 molecules. Nature, 401(6754):680–682, 10 1999.
