My iMessage is slow/delayed to a particular contact
I am having issues with a particular contact over iMessage. It seems to be really slow to send (blue bar is slow across screen), when it blue bar has cleared, delivered is delayed slightly also. However, it seems that the message is received before my phone suggests that it has been delivered. This is a hassle because the conversation does not flow, as the contact will be typing back to me before other messages have seemed to send.
It seems this all started post ios 9 update, and 9.0.1 hasn't addressed this issue.
Also, the contact turned cellular off, and back on again for other reasons and it seemed to change the settings of iMessage whereby the messages send and receive to the email of that contact and it works fine.
iPhone 5c, iOS 9.0.1