1) You have to make sure you have an SE navcore that works well with ONE models - one that is RAM friendly.
2) You need to have the right Vocalizer TTS voice that will work well with ONE models. You just copy the folder into the root of the device. I have a modified version of "Samantha" which is very clear and works extremely smoothly - no noticeable lag.
3) You need to make sure you delete the line "ttsoff" in the setconfig.txt file found in the Config folder or the root of the device.