So, I've decided to forego reactivating my home internet, on the grounds that I don't need my computer connected to the internet at all times, and can use my phone as a hotspot if I really need the PC to go online.
After about a month and a half of the computer being offline, I connected it to upload some things, and access some sites which are more trouble than they're worth on iOS. And, wouldn't you know, the change in how the computer behaves is dramatic. So much so that I expect that whatever malware is involved takes a while to notice the changes.
What changes, and why do I think the route cause is so weird? Well, I should mention that I've had the wifi on the device itself turned off from time to time without these issues immediately changing, but if it is off for a bit longer—I can't say if it's time, or number of reboots, or what—then the symptoms mostly go away.
So, what are the symptoms? I'm collecting lots of seemingly unrelated issues in this list, because they all seem to be affected by internet access.
CPU usage. The standard sign of Malware—only getting half of the exsected speed, random moments of increased latency all around, etc. After the internet is disabled long enough, these go away for the most part. As in, weeks of constant internet access can make games like The Gate and Psychostrike take so long to load that the load music/announcement repeats, but without, it loads in like 5 seconds.
audio problems, most noticeable with NVDA. I almost forgot about how persistent these use to be, since they mostly disappeared during the ofbline month+1/2. The two most common are audio hanging, repeating the last 0.1s or so until the system is restarted. It was troublesome enough getting restarting to work normally, most of the time, that I assumed the whole system was hanging and used the power button. This is not the case and my system has just made getting to one of the restart buttons needlessly difficult in most cases. I cannot think of a time in which this happened that NVDA was not speaking in the moment of the hang. It does seem to happen regardless of which synth I'm using, though.
The other NVDA-OELATED audio bug is harder to explain. Sometimes, when NVDA starts, the sound will be distorted. The degree of distootion varies wildly. In the simplest case, it sounds kinda like distootion you'd hear over radio, or on damaged tape. Other times, every sound it makes spawns this awful, ear-piercing synth sound, like something between a sawtooth and unsigned square, and it rises in pitch without ever going back down. In the worst case, which has only hapened once or twice, the sound starts and never pauses, even if there are no other ssounds playing. I remember this affecting windows sounds at least once, but i usually restart NVDA immediately, which is like re-rolling for audio stability. Which is to say, the problem sometimes goes away if I restart NVDA, but sometimes it respawns at a different level of distortion.
If I'm playing a BGT game with PS1 controllers, every few minutes, the game will slow to a crawl (through molasses) and I don't even know if it resolves itself, or if pausing and pressing buttons and switching windows has any effect. This stopped completely during the month+ without internet.
*] and here we have a problem which may or may not be related. Sometimes, Bgt gets stuck looping the last second or so of audio, and becomes unresponsive. Strangely, this only affects sounds in the sound_pool, not BGM. This happens with and without internet access, but I noticed that the frequency seemed to change during this period. it was a semi-common occurrence, always, but during the offline period, it became a daily thing, with some rare exceptions. After I turned on wifi, then turned it off again, the problem hit like 3 times in one hour. This could be a coincidence, but considering the way sound is in other problems, I'm not so sure.
I've had this issue where BGT's run function gives inconsistent results. Sometimes, run("explor].exe", filename) will open the file in the default program, and sometimes it will open Windows Explorer as though I'd passed no arguments. I have previously tested this to see if wifi made a difference, and found that it did not. However, I struggle to remember a time during the ofbline period, after the first day or so, when it did not behave correctly. After breaking the net-fast, the problem immediately returned, as inconsistent as ever.
I should add that having someone claim to do something something antivirus had no effect, other than on speed. Of course clone and reformat is a better option, but I still find this particular combination of symptoms interesting enough that I'd rather try to understand the underlying causes first.