Rails respond to JSON extremely slow on Heroku

I'm using a JSON response in my controller:

respond_to do |format|
    format.json {render json: [products: @products, boxes: @boxes,categories: @categories, giftpacks: @giftpacks,extras: @extras,user: @user,kits: @kits]}
end

In my local server, I'm getting this time load:

When deployed to Heroku, I'm getting this time load and a 2.4MB file:

¿How can I speed up things? This is so annoying.

Thanks!