[G3 Q&A, Help & Troubleshooting] HashtagFTW:
Device is a F460s (a.k.a the LG G3 Cat.6 / LTE-A version with the Snapdragon 805) currently running Mokee ROM. Was running RR 5.7.0 by JZ_shminer until today.

So basically, here's the summary of the problem. When I'm downloading very large apps / games (1GB+) over WiFi from the Play Store (N.O.V.A 3 for example), the first 30 seconds or so of the download goes as expected with speeds that are close to nearly maxing out my 100mbps connection. Unfortunately, after the initial 30 seconds of 10MB/s glory, (the time that it takes for the download speed to "throttle" actually varies quite a bit, usually the first runs last for around 30-40 seconds at 10MB/s while immediately starting the download after cancelling a download that was already being "throttled" only lasts for around 10 seconds before being "throttled" again) the speed suddenly drops to around 500KB/s for no apparent reason and stays there for the rest of the download. At first, I was pretty sure that it would be an easy fix related to some network setting I messed around with accidentally, so it was rather concerning when I saw a network settings reset didn't do anything. After resetting the network settings multiple times in hopes that it would eventually fix itself, I carried out a few basic tests to try to pinpoint the problem. Here were the tests that I did : Multiple speedtests of my connection using both the Speedtest.net Ookla app and the traditional website through chrome to confirm that the issue is only present in the Play Store (Although I will say that the speeds were rather unstable using the app, too bad they were still more than 10x of what I was getting after being "throttled" in the Play store), downloading the same app using several different phones (they were all pegged at 10MB/s, all of them), doing the same thing with AdGuard disabled (made 0 difference, thought maybe the local VPN did something), repeating all of above while using a VPN, doing a full wipe, and even installing a new ROM when that didn't work. Something that I did find intriguing though, is that using a VPN while downloading (Not turning off AdGuard, just using a third-party VPN app), got me rather surprising results. While my initial "starting" speed (basically the first 30 seconds) with a VPN was around 1/3 of what the non-VPN run would get in the first few seconds, it could sustain that 3-4MB/s for the rest of the download. All 1.3GBs of it. Would this indicate that this "download speed being throttled in the Play Store under specific circumstances" thing is a software-side issue? But if that was the case, why would installing a new ROM make absolutely no difference? Haven't tried stock MM yet though, planning to do it tomorrow to see if that makes any difference.

Also, a few things to note that could potentially help determine whether it's a hardware or a software problem : Yesterday, I finally got around to applying the dbrand skin that I bought for this phone a few weeks ago, and in the process, used a hair dryer to heat up the camera / volume and power button area for around 30 seconds to ensure a more optimal fit. Could this have damaged something inside the phone that has an impact on sustained download speeds? Personally, it seems rather unlikely, considering that it was done with the battery removed and dbrand recommends a heat gun to do the same thing.

There's also this more-likely scenario : When I woke up this morning, I immediately grabbed my phone next to me to check the time, only to find out that it was bootlooping. To be specific, it was the kind of bootloop that you encountered when you had a faulty battery, or at least I'm assuming it is because I had the exact same thing happen because of a faulty battery. The phone was also SUPER hot when I initially picked it up and no button combination seemed to able to get it out of the bootloop, so I had to peel off my poor literally-just-applied-the-day-before dbrand skin in order to remove the battery and cool it down. After a few minutes of leaving the phone to cool down by itself, I inserted the battery back in and to my surprise, everything seemed to be in working order. There were no signs of any sort (until now that is) that indicated anything was wrong with the phone, so naturally, I assumed it was fine and chalked it up to my very stupid and dangerous habit of "leaving the phone in bed while fast-charging with a third party Quick Charger" which led to the phone overheating to the point of forced shutdown and reboot, only to fail to do so repeatedly...? I don't know if that even made any sense, as one would assume that a phone reaching highly dangerous temps would shut itself off and STAY off in order to actually prevent any damage being done to the components. I don't know much about this stuff though.

tl;dr : Phone is getting painfully slow download speeds all of a sudden and it's not my connection, 99% sure. Not sure if hardware or software problem. Leaning more towards hardware atm. Although I'm not sure how that would be possible. It would be great if you read through the whole thing, as there is quite a bit of info, but really any help would be greatly appreciated.