Is this just for video, or have you tested it through other methods like speedtest.net or big software updates?
Some ISPs will throttle video streaming using various methods, but through a VPN they will not be able to tell when something is video vs anything else.
You should probably also change your DNS server to something outside of your ISP's control, ideally encrypted using either DoT (DNS over TLS) or DoH (DNS over HTTPS).
When using a VPN like Proton, I believe you are already using the VPNs DNS
Yes, sorry, I meant for when off VPN. When you're on Proton VPN, it should use Proton's DNS server by default.
Tracking and controlling DNS queries is one way ISPs can classify and direct traffic, so it's possible that using encrypted DNS would help without using a VPN. Not really sure how common this is in practice.
It's a good default to use something like DOH (DNS over HTTPS)
Since the content of the websites is encrypted, it only makes sense to also encrypt DNS as a baseline
OFC it's not as comprehensive as a VPN or Tor, but is a sensible default for day to day stuff
When on the VPN, you're actually going from your home, to your ISP, and then your ISP forwards your stuff to the VPN provider and then once it's with them, they move it out onto their ISP until it reaches the final destination. You're adding more hops even if choosing a local region/point-of-presence.
Like other person said, I have definitely heard of how ISPs would throttle some traffic and in the past, this would actually be done for VPN traffic--not really your regular traffic. Not too sure about why the VPN speed test is giving those numbers but there's probably something not being taken into account.
Also not all VPNs are the same. I think Opera was flaunting having a built-in VPN at one time and this really wasn't a VPN IIRC. Never tried Proton VPN and so I can't tell.