Lastly, you might want to contact your VPN provider and/or casino if the issue persists after several efforts to resolve it. I am sure they will have one or two to say. In my view though, I don't think casinos will deliberately slow down your VPN, they will rather warn you or block the account if they do not allow it.
Seems like it's back to normal for now. I didn't ask them yet but it's probably a temporary problem with their servers or DNS resolver. They do have options to block gambling websites but I don't think it slow down access for websites even if we don't activate it. My ISP do block gambling websites so that's that. I believe Mullvad will be transparent if they changed anything so I'll try to see if anything changes in the near future.