I have experienced it in several sites but never have I tried it in any casino before since I don’t want to use vpn and out my account at risk, even sometimes I’m unable to visit or open sites like Reddit and quora but google search works very fast and when I change the network provider or turn on vpn everything becomes normal.
Can you access Reddit or Quora without a VPN? Sounds like a DNS issue to me. One of the DNSs my provider uses is very slow when responding to requests to social media, but they don't necessarily block them. Maybe try changing your DNS to see if it fixes your issues. Sometimes it can also help bypass your ISP blocklist especially when your country doesn't block websites you're trying to access. I am not sure if VPN-friendly casinos allow us to use that though, you better ask their support if you can't find it on their ToS. CMIIW.