Did this start happening right after the update? What do you battery stats show?
Excessive heat is caused by one (or a combination) of three things: High CPU use, rapid battery discharge or defective circuitry.
The first comes from a lot of active use or an app that has gone "rogue" (won't shut down when not in use). A simple reboot of the phone can solve this most of the time. The battery discharge is sort of related to the CPU use, but heavy Wifi or streaming (aka casting) can cause that, too without a heavy CPU load. The last, well, it could be a battery about to flame out, but LG batteries don't come from Samsung

or it could be failing radios or a short on the motherboard. If it's the latter of those, then you'd probably see some things simply stop working.
If you say the phone works but is laggy and hot, that makes me think it's a heavy load. You could try and troubleshoot but I've see people put many many hours in and still not find the culprit. I would recommend a full backup of everything (you do that anyway, right?) and then a factory reset. That should take you two or thee hours from start to finish.
If after all that, it still gets hot and lags, i think you got a bad phone. It happens. The good news is that LG offered an extra year warranty for free when you bought it. Please tell me you opted in for that.
Oh, one other thing could make your phone hot ... putting it in the microwave for a minute on high, but I'm guessing you didn't do that.