I know the problem with slow VPNs in China and 4K IPTV buffering. Many servers are overloaded and have high latency.
Better performance usually comes from using WireGuard or OpenConnect. Streaming optimized servers or SmartDNS can also improve speeds a lot.
Another option is renting a VPS in Japan Singapore or Hong Kong and setting up WireGuard yourself. This avoids crowded public VPNs and gives better bandwidth.
Some VPN providers offer China optimized routes that reduce lag and packet loss.