Laravel app slows down on select pages after PHP 8 upgrade (VPS with Webmin)

When upgrading a Laravel application from PHP 7 to PHP 8, you might encounter performance issues on certain pages. This issue can be caused by several factors, including but not limited to, Laravel's outdated dependencies, incompatible extensions, and configuration settings. In this answer, we will discuss the possible causes and solutions to help you optimize your Laravel app for PHP 8.

Outdated Dependencies: Laravel relies on several third-party packages to function correctly. When upgrading PHP, it's essential to ensure that all dependencies are up-to-date. You can update your composer dependencies by running the following command in your terminal:
```
composer install --no-autoload
```
This command will update all your dependencies and rebuild the autoload files. If you encounter any issues during the update process, you can try updating individual packages by running:
```
composer require package-name/package-name --with-all-dev
```
Replace "package-name/package-name" with the name of the package you want to update.
Incompatible Extensions: PHP 8 introduces several new features and deprecates some old ones. Some Laravel extensions or third-party packages might not be compatible with PHP 8. To check for incompatible extensions, you can run the following command in your terminal:
```
php -m
```
This command will list all the loaded extensions. Check for any extensions that are not compatible with PHP 8 and remove or replace them.
Configuration Settings: Laravel's configuration settings might need to be adjusted when upgrading to PHP 8. For instance, Laravel's default file upload handler, "Symfony\Component\HttpFoundation\File\File" is no longer supported in PHP 8. You can change the default file handler by updating the "filesystems.disks.local.driver" setting in your ".env" file:
```
filesystems.disks.local.driver=local
```
Additionally, Laravel's default session driver, "session.store" might not be compatible with PHP 8. You can change the session driver by updating the "session.store" setting in your ".env" file:
```
session.store=session.store.file
```
Optimize Laravel for PHP 8: To optimize Laravel for PHP 8, you can enable Opcode caching and use Laravel's built-in caching mechanisms. Opcode caching stores the compiled PHP code in memory, reducing the time it takes to load the pages. You can enable Opcode caching using Webmin or your preferred PHP configuration file.

Additionally, you can use Laravel's built-in caching mechanisms like Redis or Memcached to cache frequently accessed data, reducing the number of database queries and improving performance.
Monitor Performance: After making the necessary changes, it's essential to monitor your Laravel app's performance regularly. You can use tools like New Relic, Blackfire.io, or Laravel's built-in profiler to identify performance bottlenecks and optimize your app accordingly.

In conclusion, upgrading a Laravel app to PHP 8 can lead to performance issues on certain pages. To optimize your Laravel app for PHP 8, you need to update dependencies, check for incompatible extensions, configure settings, and optimize your app using caching mechanisms. Regularly monitoring your app's performance is also crucial to ensure optimal performance.