Should dielectric constant be high or low for high-speed design? And why?
I'm very confused that dielectric constant should be selected high or
low to design high-speed boards.

I'll try and answer your question by considering inter-chip communication speeds on a circuit board using copper tracks. With a higher dielectric constant and a given PCB track width, the capacitance will be higher and this inevitably means that the characteristic impedance of a track will be lower - this has a knock-on effect on speed. The velocity factor (the ratio of actual signal propagation speed to the speed of light) is given by this equation: -
$$VF = \dfrac{1}{\sqrt{μ_r ε_r}}$$
Pretty much all PCBs I've come across have μr = 1 (magnetic permeability) and so the equation effectively becomes: -
$$VF = \dfrac{1}{\sqrt{ε_r}}$$
So, with a higher electric permittivity (aka dielectric constant), the signal propagation speed relative to the speed of light (VF) lowers. In simple terms, the speed reduces.
So, if inter-chip communication speed is of importance you try and reduce the effects of a higher dielectric constant by making track widths smaller so there is less distributed capacitance along the track carrying the important signal.