There are a few factors on why you may perceive that the voice quality is poor.
First thing to check is if the voice you that you have set on the iOS Speechify app is on the Premium voices, To learn how to change the voices, click here.
You can also try to switch to another HD voice to check if it is also happening on another HD voice. If it is, try to double check your internet connection again. You can also try to restart the app, by killing the app and relaunching it again.