A proton and an electron are released from rest at the origin in positive x-direction where electric potential increases along the x axis.

What can you say about their change in speed?

the speed of proton increases and the speed of electron decreases

As positive charge moves from a region of high electric potential to a region of lower electric potential, its potential energy decreases and kinetic energy increases to conserve energy and thus, speed increases and vice versa for negative charges.