HTC One M8 :: How To Fix A Single Contact On Stock SMS App

This started happening pretty much immediately since I installed the 4.4.3 update late July, and has just gotten worse.

For a single one of my contacts, the SMS app lags HORRIBLY whenever I am doing anything with said person. It takes about half a minute just to load when I click his name, and the phone keeps completely freezing for up to 10-15 seconds EACH letter I type! It's not just the SMS app itself, but even the keyboard freezes up and is unresponsive. I can keep typing and it will apparently actually record the next 30 or so taps and just blaze through them ... then lag for another half minute or so again. None of the settings (how little there are) have had any effect. Once I click send the message also takes a bit before it appears and starts to send. Any action with this contact open in the SMS app seems to lag my phone like a unusable nightmare.

Since this is the person I tend to text the most, I have been tempted to crush the phone in my hand at this point. It is ONLY happening with them, and ONLY after the 4.4.3 update. It's not consistent either; I was still having this issue earlier today, but right now as I wanted to text him again it was working perfectly lag-free.