Throttling on VPN?

Hi, for a while now I've been having issues with my speed randomly dropping to 10 mbps from around 350. It happens only when I'm connected to my VPN (ProtonVPN) and was wondering if there was any kind of throttling happening?

Two websites I've found that cause this every single time are twitch.tv and decathlon.co.uk, whenever I start to load these websites my speed instantly drops.

Normal speed:

When on Twitch:

The speed goes back up to normal 15-30 seconds after leaving the websites.

And here's when I start to load decathlon.co.uk too:

I've been in contact with the VPN company and it seems like it isn't on their end, I don't know how long it has been happening for (I guess around a month and a half). I did have my Superhub replaced a few months (?) ago. I've tried in router mode, in modem mode with another router and still have the same issue.

Any idea what's happening?

Thanks.