Throttling... or something else?

So, I'm fairly certain that my ISP is throttling YouTube, at a minimum. But, I wanted to pick some brains and see if there are any other possible causes.

I'm on a 50Mbps connection, and get close to that on Speedtest. When I try to load a YouTube video, it takes 10-15 seconds for it to start playing, and it auto-detects at 240/360. It won't run any faster without severe buffering time.

Now, I load up NordVPN. Speedtest reports speeds at 40Mbps through the VPN, which is expected. With the VPN active, when I load a YouTube video, it starts playing immediately and smoothly at 1080p at a bare minimum.

On the other hand, when I load a video on Vimeo, regardless of the VPN being active or not, I can stream it at 1080p+ with no buffering.

Should also note, I'm the sole user on the WiFi during these tests, so it's not other people or devices sucking up bandwidth locally.

To me, this just screams that YouTube is being throttled, but is there any other explanation I'm overlooking?

System Specs? Happens the same on both of my machines, both WIndows 10; a Lenovo E570 w/ an i7 and 16GB Ram and a PC I built a while back with an FX8350 and 8GB Ram.