Old Android Phone Permanently Slow?

Currently I have a new phone because of this. But I also have an HTC Desire EYE (M910x, Lollipop 5.0.2) and ever since I forwarded a video, MMS, from and taken right from my dad's iPhone 6S to mine, opened it after a stagefright warning (on my custom texting app Textra), this phone has been so slow, laggy, hardly responsive and to move anything on the screen properly would need to be done in a snail pace, as the animations are, too. Before that moment, this phone had been perfectly fine, and it was just about paid off, too, after the 2-year contract. Now, it is in a state of random flashes of full screen static every now and then, and even after factory resetting every thing, wiping it all, it was slow on the initial setup. I do not know how this is possible, and can't find anything related anywhere because it is so broad to search "Android phone lagging even after factory reset" where only the first few words actually have results.

Factory reset the phone. If not send it for repair.