We are scaling a Next.js application experiencing significant traffic spikes. While ISR (Incremental Static Regeneration) and standard edge caching help, we face challenges with cache invalidation for dynamic user-personalized content and latency in cold serverless functions. 

What architectural patterns or caching strategies (e.g., Redis layer, stale-while-revalidate, edge middleware) have you found most effective to maintain sub-100ms response times without serving stale transactional data?