shinyApp(

  fluidPage(
    sidebarLayout(
      sidebarPanel(
        sliderInput("n", "Number of points", 4, 32, value = 8, step = 4)
      ),
      mainPanel(plotOutput("plot"))
    )
  ),

  function(input, output, session) {
    output$plot <- renderCachedPlot(
      {
        Sys.sleep(2)  # Add an artificial delay

        rownums <- seq_len(input$n)
        plot(cars$speed[rownums], cars$dist[rownums],
            xlim = range(cars$speed), ylim = range(cars$dist))
      },
      cacheKeyExpr = {input$n }
    )
  }
)