Performance problems after launch usually come from a combination of small decisions, not one single bottleneck.
Check first
- TTFB and server-side caching
- Image sizes and lazy loading
- Number of third-party scripts
Then inspect infrastructure
- CPU and RAM during peak hours
- Disk subsystem limits
- Queues, cron jobs and background tasks
Fast wins
- CDN for static assets
- Page cache for CMS workloads
- Moving from overloaded shared hosting to VPS when needed
Site speed is not one trick. It is a sequence of smart checks on top of stable infrastructure.