When it comes to enhancing the performance of your Ruby on Rails application, leveraging the right gems can make a significant difference. Here are 10 essential gems that are widely regarded for optimizing application performance:

1. Bullet: This gem helps to reduce N+1 queries by eager loading associations, ensuring that your database interactions are efficient and fast.
2. Rack::Cache: A powerful rack middleware that adds caching capabilities to your Rails app, reducing response times and server load.
3. skylight: A performance monitoring tool that provides insights into your app's performance, helping you identify slow requests and optimize them.
4. Rollbar: This gem helps in tracking errors and exceptions in real-time, allowing you to quickly resolve performance bottlenecks.
5. Memcached or Redis: Both are essential for caching data and improving response time by storing frequent queries and results.

Continuing with our list, here are the remaining gems you shouldn't overlook:

6. Rack::Deflater: A middleware that compresses your app's responses, reducing bandwidth usage and improving loading speeds.
7. ActiveRecord::ConnectionAdapters::ConnectionPool: This gem ensures that your database connections are efficiently managed, optimizing resource usage.
8. Jbuilder: Jbuilder provides a fast and flexible way to build JSON responses, enhancing the speed of API interactions.
9. ActsAsTaggableOn: If your application uses tagging, this gem optimizes the tagging process to enhance data retrieval performance.
10. Ahoy: A lightweight tracking solution that helps in monitoring visitor behavior without impacting performance.

Common Performance Pitfalls in Ruby on Rails can severely hinder the efficiency and responsiveness of your web applications. One major issue is N+1 queries, which occurs when an application executes a separate database query for each record instead of using a single query to retrieve all associated records. To avoid this pitfall, leverage ActiveRecord's includes method to pre-load associations and minimize database round trips. For instance, instead of fetching user posts one by one, you can fetch all users and their posts in a single query, leading to significant performance improvements.

Another common problem is inadequate caching strategies. Without proper caching, your application might generate excessive load on the server, particularly if it processes the same request multiple times. Implementing fragment caching or action caching can drastically reduce the amount of work done by the server. For example, you can use cache do blocks around sections of your views that don’t change often, allowing Rails to serve cached content efficiently. By identifying and optimizing these aspects, developers can avoid performance bottlenecks and enhance user experience.

Measuring the speed of your Rails application is crucial for ensuring optimal performance and user experience. Start by using profiling tools such as Rack Mini Profiler or Bullet to identify bottlenecks in your code. These tools can give you insights into database queries, rendering times, and memory usage. Additionally, consider implementing application performance monitoring (APM) solutions like New Relic or Skylight for real-time tracking of key metrics. Once you have a clear understanding of your app's current speed, you can proceed with optimization steps.

To improve your Rails app's speed, focus on code optimization and caching strategies. Start by reviewing your controllers and views for any redundant code and unnecessary database calls. You might consider using fragment caching for static portions of your views, and employ Russian doll caching for complex nested components. In addition, implement background processing using tools like Sidekiq to offload time-consuming tasks from your web requests. Follow these steps consistently, and regularly measure the performance improvements to keep your application fast and efficient.