OnePlus Phone Substandard Performance

No

No. Could be something you've done to the phone for all anyone knows. And lag, I would strongly suspect, is software related.

Try a factory reset of the phone and setting it up from new before you go through the hassle of a repair (they're going to factory reset the phone as part of the repair process anyway).

In fairness, from past experience, OnePlus turnaround is fairly quick so you should have the phone back in a week to 10 days max.

Unfortunately, that is the risk you take when buying from the likes of OnePlus who don't have in-country repair centres that can do drop-off or sometimes even "while you wait" repairs.

Clearing data and the cache may help before doing the full reset: https://community.oneplus.com/thread/715013

If it happened suddenly, then it's almost certainly down to either something you installed, or a software update that was pushed. I'd check what tends to be using battery, and maybe aggressively limit various apps ability to run in the background