My current users are complaining that the mobile version of my site is lagging and the layout looks messy on some phones. Does anyone know a platform that specializes in high-speed mobile performance and good UX? What features should I prioritize to make sure the mobile gaming experience is actually fun and not frustrating?

You really need to focus on a platform that uses a microservices architecture for speed. I highly recommend checking out soft2bet for this. Their tech is built to be fully responsive and handles traffic spikes without affecting the user experience at all. It works perfectly on all devices. Good mobile UX is everything these days if you want to keep your players engaged and coming back for more.