R Shiny: fast reactive image display

I'm trying to display images in my shiny app reactively. I've successfully done that in the server.R script with:

output$display.image <- renderImage({

    image_file <- paste("www/",input$image.type,".jpeg",sep="")

    return(list(
      src = image_file,
      filetype = "image/jpeg",
      height = 520,
      width = 696
    ))

  }, deleteFile = FALSE)

BUT it's very slow.

However, it is VERY fast to embed one of the images into the ui.R script like so:

tabPanel("Live Images", img(src = "img_type1.jpeg"))

Why is there such a difference? Is there any way to make the reactive images appear faster?