Slow shinyalert with large memory and callback

app.R

library(shiny)
library(shinyalert)

options(digits.secs=6)

ui <- fluidPage(
   useShinyalert(),
   
   actionButton("foo","Modal With Callback"),
   actionButton("bar","Modal Without Callback"),
   
   sliderInput("sz", "Matrix Dimension:", 10, 20000, 1000),
   h3("Matrix size:"),
   textOutput("matrix.sz"),
   h3("Modal response time:"), textOutput("modal.time")
)

server <- function(input, output) {
  modal.time <- reactiveVal()

  ok <- function(x) message(x)
   
  observeEvent(input$foo, {
    t1 <- Sys.time()
    shinyalert("With Callback", callbackR=ok)
    modal.time(Sys.time()-t1)
  })

   observeEvent(input$bar, {
     t1 <- Sys.time()
     shinyalert("Without Callback")
     modal.time(Sys.time()-t1)
   })
   
   output$matrix.sz <- renderPrint({ print(object.size(z()), units="auto") })
   output$modal.time <- renderPrint({ modal.time() })
   
   z <- reactive({
     matrix(nrow=input$sz, ncol=input$sz)
   })
}

# Run the application 
shinyApp(ui = ui, server = server)

Regarding shinyalert issue #19.