FINALLY, I have a solution! Woohoo! ^_^
There is an app called "Sms quick delete" that works! Granted, it took me 3 hours to delete the thread that is now 4500+, but at least it worked! I set it to never turn off, kept it plugged in, and just let it sit uninterrupted, checked on it 3 hours later, and it was fine
another thing that could have attributed to it actually working was that I deleted most of my smaller threads with other people too, I believe that could have sped up the process a bit as well. Anyway, give it a shot! Hope it works for you too!! I know how ANNOYING it was texting my girlfriend, whom I text the most, and having to wait 1-2 minutes every time I press "send" and the general lag when I am inputting text as well!