title: R shiny DT slow
body: <p>I like the features of <code>DT</code> package, it addresses most of my issues. One thing I noticed with <code>DT::renderDataTable</code> compared to previous one is that it is very slow when <code>filter=top</code> is used. I understand that it has to gather the values for factor columns and date columns, but is defeating the purpose. It feels like I have to chose between these additional features vs speed of rendering the page. </p>
  
  <p>Is this normal or is there any other way to improve the speed of rendering the page while using DT with filter=top?</p>
  
  <p>NOTE : I'm using that on 32 column 350k row data table</p>
  
  <p>Here is the structure of rb:</p>
  
  <pre><code>Classes ‘data.table’ and 'data.frame':  357574 obs. of  32 variables:
    $ 1col : chr  "00000001175" "00001050760" "00002020102" "00002050202" ...
    $ 2col : Factor w/ 6 levels 
    $ 3col : Factor w/ 22116 levels 
    $ 4col : Factor w/ 4 levels 
    $ 5col : Factor w/ 22 levels 
    $ 6col : Factor w/ 11 levels 
    $ 7col : Factor w/ 2 levels 
    $ 8col : Factor w/ 7 levels 
    $ 9col : Factor w/ 2 levels 
    $ 10col: Factor w/ 2 levels 
    $ 11col: Factor w/ 114 levels 
    $ 12col: Factor w/ 2 levels 
    $ 13col: Factor w/ 258 levels 
    $ 14col: Factor w/ 27 levels 
    $ 15col: Factor w/ 32 levels 
    $ 16col: Factor w/ 314 levels 
    $ 17col: Factor w/ 2751 levels 
    $ 18col: Factor w/ 7 levels 
    $ 19col: Factor w/ 3952 levels 
    $ 20col: Factor w/ 4 levels 
    $ 21col: Factor w/ 5389 levels 
    $ 22col: Factor w/ 11 levels 
    $ 23col: Factor w/ 27 levels 
    $ 24col: Factor w/ 22 levels 
    $ 25col: Factor w/ 2491 levels 
    $ 26col: Factor w/ 113 levels 
    $ 27col: Factor w/ 258 levels 
    $ 28col: Factor w/ 32 levels
    $ 29col: Factor w/ 6 levels 
    $ 30col: chr  "ABC" "CDE" 
    $ 31col: Factor w/ 2679 levels 
    $ 32col: Factor w/ 8333 levels 
  </code></pre>
  
  <p>As a test, I converted any factor column above that has more than 100 levels to character format and that improved rendering. Rendering time came down from 40 t0 6secs</p>