You can download enhanced quality voices for iOS to improve the quality of the speech. In Settings, go to General > Accessibility > VoiceOver > Languages & Dialects > Add New Language... to add a new language.
When your device is plugged in to power and on a wifi network, tap the More Info button on your newly added language to see a switch that will allow you to enhance the quality of the voice. Setting the switch to On will download some data to improve the quality of the voices used by iOS.
You can also adjust the speaking rate on this screen, but this will only apply to VoiceOver.
When VoiceOver is not being used, the speech features in Conjugations ES use a non-adjustable speech rate that's comfortable for non-native speakers of Spanish.
Apple has a support article about voices at https://support.apple.com/kb/HT5025. We hope it will be kept up to date as Apple introduces new changes and features to the operating system.