According to de Broglie's hypothesis, the wavelength of an electron is inversely proportional to its momentum (and hence speed), given by $\lambda = \frac{h}{mv}$. When the speed ($v$) of the electrons is increased, their de Broglie wavelength ($\lambda$) decreases. The angular width of the central maximum in a single slit diffraction pattern is directly proportional to the wavelength, given by $2\theta = \frac{2\lambda}{a}$, where $a$ is the slit width. Therefore, a decrease in wavelength results in a decrease in the angular width of the central maximum.