What vpn are you using? Have you tried contacting their support? Try connecting to the nearest servers to your true location, at least that's what I do with Nordvpn.

Which vpn provider are you using and on what hardware? You need a) a decent vpn provider with lots of high bandwidth egress points and b) enough processing power to run the vpn and IPTV client at the same time. For example, I use private internet access and a 4K Amazon firestick on Virgin and it works fine.

Try different gateways

If you are using express vpn app you could try an openvpn app instead

Try different servers from your VPN provider. I had to find a faster server to stop the buffering on my setup.

Since a VPN server encrypts your live stream and sends it to you, that in itself creates an extra possible point of failure and headaches. I currently seed over 1000 torrents on my home seed box for many years. I have never received a warning letter from Frontier FIOS or experienced any throttling. I will never use a VPN unless it’s required. Furthermore, I never heard of anyone in the USA getting in serious trouble for subscribing to IPTV services as an end user. My IPTV streams play just as good as Sling or any mainstream providers do. If you must use a VPN don’t go cheap and select a budget provider. You get what you pay for. Also select the server with the shorter route to you and your IPTV service. You don’t want to be routing your connection through the UK if your stream originated in the USA for example. Using trace route could help you analyze that. Always run speed tests on your VPN provider to see if they match your home speed. Example, If you get 100 mbps download service at home and your VPN provider is only sending you data at 20 mbps in a speed test, drop it and find another server/provider.

Will try some different servers on express vpn and will keep you posted. Many thanks

Don't user any encryption - cipher=none.

No need to encrypt the IPTV stream.