[JaxChris]

Inside HTC messaging or Windows SMS? If it's inside HTC messaging then try deleting the message history for that contact. The longer the saved conversation the longer it takes to "send" each new message. The delay before it shows up in the conversation and then actually sends it can get very long if you have a lot of SMS messages on the device already.

If it is within Windows SMS, I would suggest a hard reset/update profile/update PRL (I know PRL has nothing to do with it); just to see if the issue resolves. Microsoft MyPhone is your friend in backing up your PIM data for easy restoration.