Next.js : Performance Issues with Large Codebase Using App Router

I’m experiencing significant performance issues with my Next.js application. My project has grown substantially, and the build times and page loads have become noticeably slower. Here are the details of my setup:

Next.js Version: 15rc
Router: App Router
Codebase Size: Large with multiple complex pages and components

Problems:

Build Times: The build times have increased drastically as the codebase has grown.
Page Loads: Initial page loads are slow, and navigating between pages isn't as smooth as expected.

What I’ve Tried:

Code Splitting: I've implemented dynamic imports for some components, but the improvements are marginal.
Caching: Enabled server-side caching, but it hasn't significantly reduced the load times.
Analysis Tools: Used Webpack Bundle Analyzer to identify large bundles, but refactoring them hasn't helped much.
Optimization Tips: Followed various optimization tips from the Next.js documentation, but the performance gains are minimal.

Questions:

Are there specific configurations or best practices in Next.js 15rc that are particularly effective for large codebases?
How can I optimize the App Router for better performance in a large application?
Are there any known issues with Next.js 15rc that could be contributing to these performance problems?
Would upgrading to a stable release (if available) improve performance, or are there other recommended strategies?

Any advice or guidance on how to tackle these performance issues would be greatly appreciated!

Thank you!