how to initialize shiny::reactive() with default filter options?

In my shiny app I have a bunch of reactive dataframes that can be filtered. For example in server:

panel2_data <- reactive({
    df <- iris %>% 
      filter(input$year_range_slider) %>%
      group_by(Species) %>%
      summarise(Petal.Length)
    return(df)
})

When I launch my app, all those reactive dataframes are calculated at the same time and it freezes the app for several seconds. How do initialize reactive so that it only is calculated when the initial filter options are changed?

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