Local assets precompilation on Rails 4 using Capistrano 3

deploy.rb

# also you need to uncomment next line in Capfile
# require 'capistrano/rails/assets'

namespace :deploy do
  namespace :assets do

    Rake::Task['deploy:assets:precompile'].clear_actions

    desc 'Precompile assets locally and upload to servers'
    task :precompile do
      on roles(fetch(:assets_roles)) do
        run_locally do
          with rails_env: fetch(:rails_env) do
            execute 'bin/rake assets:precompile'
          end
        end

        within release_path do
          with rails_env: fetch(:rails_env) do
            old_manifest_path = "#{shared_path}/public/assets/manifest*"
            execute :rm, old_manifest_path if test "[ -f #{old_manifest_path} ]"
            upload!('./public/assets/', "#{shared_path}/public/", recursive: true)
          end
        end

        run_locally { execute 'rm -rf public/assets' }
      end
    end

  end
end

Uploading many files (and some non compressed) can be slow.
Try compressing and packaging for uploading:
[Example here] (https://gist.github.com/jairud-garcia/4e9210c4d896b7c56f5fd54164812b8f)