How can I slow down podcast playback speed to 90% - 75% on my iPhone?
Apple's Podcasts app lets you slow down playback speed only to 50%. To slow down the playback speed, tap the indicated button to cycle between 1x, 1.5x, 2x and .5x speed.
There are various 3rd party Podcasts app for iOS which lets you slow down the playback speed with much fine grained values. One such app is Podbean Podcast App & Player. It lets you slow down the playback speed with granularity of 10%.