Rails 4.2 app very slow in development - ruby-on-rails

Question

I've been banging my head against the wall on that one for a couple of days now.
My app is running super slow in development mode. A good amount of time is spent on serving the assets (especially images). Each request is extremely slow, taking up to 7s to load a simple 50kb jpeg. Total page load time can be up to a couple of minutes...
I suspect there is a link with auto-loading of the code, since when I turn on classes caching my stuff is loaded much much faster (a few hundred ms maximum). That's not a solution though, since I need to be coding, I can't restart the server everytime I make a change.
Here's my config/development.rb
Rails.application.configure do
# Settings specified here will take precedence over those in config/application.rb.
# In the development environment your application's code is reloaded on
# every request. This slows down response time but is perfect for development
# since you don't have to restart the web server when you make code changes.
config.cache_classes = false
# Do not eager load code on boot.
config.eager_load = false
# Show full error reports and disable caching.
config.consider_all_requests_local = true
config.action_controller.perform_caching = false
config.reload_classes_only_on_change = true
# Don't care if the mailer can't send.
config.action_mailer.raise_delivery_errors = false
# Print deprecation notices to the Rails logger.
config.active_support.deprecation = :log
# Raise an error on page load if there are pending migrations.
config.active_record.migration_error = :page_load
# Debug mode disables concatenation and preprocessing of assets.
# This option may cause significant delays in view rendering with a large
# number of complex assets.
config.assets.debug = false
# Asset digests allow you to set far-future HTTP expiration dates on all assets,
# yet still be able to expire them through the digest params.
config.assets.digest = true
# Adds additional error checking when serving assets at runtime.
# Checks for improperly declared sprockets dependencies.
# Raises helpful error messages.
config.assets.raise_runtime_errors = true
# Raises error for missing translations
# config.action_view.raise_on_missing_translations = true
config.action_mailer.default_url_options = { host: '127.0.0.1', port: 3000 }
# email sending process
config.action_mailer.perform_deliveries = true
config.action_mailer.delivery_method = :test
# Defaults to:
# config.action_mailer.sendmail_settings = {
# location: '/usr/sbin/sendmail',
# arguments: '-i -t'
# }
config.action_mailer.perform_deliveries = true
config.action_mailer.raise_delivery_errors = true
config.action_mailer.default_options = {from: 'no-reply#example.com'}
config.web_console.whitelisted_ips = '0.0.0.0/0.0.0.0'
end
and initializers/assets.rb
# Be sure to restart your server when you modify this file.
# Version of your assets, change this if you want to expire all your assets.
Rails.application.config.assets.version = '1.0'
# Rails.application.config.assets.precompile += %w( jquery.Jcrop.min.scss )
# Rails.application.config.assets.precompile += %w( jquery.Jcrop.min.js )
# Rails.application.config.assets.precompile += %w( flat-ui.css.bak )
# Rails.application.config.assets.precompile += %w( bootsrap.css.map)
# Add additional assets to the asset load path
# Rails.application.config.assets.paths << Emoji.images_path
# Precompile additional assets.
# application.js, application.scss, and all non-JS/CSS in app/assets folder are already added.
# Rails.application.config.assets.precompile += %w( search.js )
Dir.glob("#{Rails.root}/app/assets/images/**/").each do |path|
Rails.application.config.assets.paths << path
end
Any insight would be much appreciated!

Answer 1

Many issues can happens in your application, for example it can be N+1 queries or some slow code, HTTP queries into other site or big assets bundle. So I suggest use profiler it will show bottleneck in your code. See list popular profilers below:
https://github.com/glassechidna/stackprofiler
https://github.com/ruby-prof/ruby-prof
Other helpful links you can find here:
https://awesome-ruby.com/#-profiler-and-optimization