title: geom_rect + coord_map = very slow
body: <p>I am plotting a map of Norway with an area of interest highlighted with a red rectangle using <code>ggplot2</code>. If I omit the <code>geom_rect</code> or <code>coord_map</code>, the map plots very quickly (&lt; 1 seconds). If I use both - which I need to - it is extremely slow to print and render (about five minutes).</p>
  
  <p>I presume this is something to do with the munching - projecting the rectangle onto the new coordinate system. Is there a way to control this?</p>
  
  <pre><code>library(ggplot2)
  library(maps)
  library(mapdata)
  
  
  xlim &lt;- c(5, 10)
  ylim &lt;- c(60, 62)
  
  norwaymap &lt;- map_data("worldHires", "Norway")
  a &lt;- ggplot(norwaymap, aes(x = long, y = lat, group = group)) +
    geom_polygon(colour = NA, fill = "grey60") +
    geom_rect(xmin = xlim[1], xmax = xlim[2], ymin = ylim[1], ymax = ylim[2], 
      colour = "red", fill = NA) +
    coord_map(xlim = c(3, 33), ylim = c(57, 72))
  print(a) # super slow
  </code></pre>
  
  <p>Using the low resolution map makes the map plotting much faster (about 10 seconds).</p>