Podcast Speed on iPhone
Yes - you can slow down podcasts by pressing the 1x button to the left of the rewind by 15 seconds button:



In the above screenshot I have pressed the 1x button until it has become ½ x button.

If the voices are "too fast to be understood," then you may have actually increased the playback speed (2x or 3x), and may want it back at 1x. Either that or their voices are actually that fast and you will want to slow it down.