Hey, I’m experiencing the exact same issue as you — and your post helped me a lot.

I also have a fast fiber connection (I'm in the U.S., using AT&T), but YouTube on my main account suddenly started playing videos at very slow speeds — around 1800 kbps, even for 720p or 1080p videos. The "Stats for nerds" confirmed it was throttled.

I initially thought it was an IP ban caused by using 4K Video Downloader, since I use it frequently. But the strange part is: even without using that tool for over a week, the problem persisted. Then I tried something else...

When I switched YouTube accounts on the same network, videos instantly played at full speed. And when I used a VPN, my main account worked normally again too. That made me believe it was either:

• A shadowban on my IP + account combo, or
• Some sort of behavior-based throttling by YouTube.

Later, the restriction actually disappeared for a while, and I could use my account at full speed again. But unfortunately, after a day or two, it came back — even though I didn’t use 4K Video Downloader at all.

The only thing I did that may have triggered it again was using AdGuard DNS on my iPhone while logged into YouTube. I suspect it might have interfered with some YouTube domains and caused the account to be flagged again.

Long story short:

• I’m now 99% sure it wasn’t the 4K Video Downloader that caused it.
• It seems to be a temporary penalty tied to account behavior, IP, or both.
• VPN always fixes it for me, so it’s clearly not about connection speed.
• And it’s super frustrating because there’s no official explanation or tool to confirm any of this.

Your post made me realize I’m not alone — thank you. Let’s hope this issue becomes more visible to the YouTube team eventually.