Hi Everyone! I have a question about architecting my app. (I'm quite new in thinking about large app architecture)

Currently I have a single (massive) Next.js 12 app that has websites for multiple domains. This is because all of these domains have similar website and share many pages and components. Currently what it does is check domain client-side and render website accordingly (themes & content as per the domain). But as it is growing, its becoming slow. What would be a better way to build websites like these. What is the standard practice in this case?