title: csv & xlsx files import to pandas data frame: speed issue
body: <p>Reading data (just 20000 numbers) from a xlsx file takes forever:</p>
  
  <pre><code>import pandas as pd
  xlsxfile = pd.ExcelFile("myfile.xlsx")
  data = xlsxfile.parse('Sheet1', index_col = None, header = None)
  </code></pre>
  
  <p>takes about 9 seconds.</p>
  
  <p>If I save the same file in csv format it takes ~25ms:</p>
  
  <pre><code>import pandas as pd
  csvfile = "myfile.csv"
  data = pd.read_csv(csvfile, index_col = None, header = None)
  </code></pre>
  
  <p>Is this an issue of openpyxl or am I missing something? Are there any alternatives?</p>