Hi, I am currently studying this topic. From my knowledge, the speed should be constant, here's why.
The ripple tank can be used to observe waves. A rod at the end is used to create ripples by vibrating it. This causes the ripples (parallel lines) to travel at constant speed which can be seen using a bright light source. The water does not travel through a medium which could be another reason for its constant speed. The parallel lines are known as wavefronts. Hope this helps.