title: Why polars.scan_csv is even faster than disk reading speed?
body: <p>I am testing <em>polars</em> performance by <code>LazyDataFrame</code> API <code>polars.scan_csv</code> with <code>filter</code>. The performance is much better than I expect. Filtering a CSV file is even faster than the disk speed!  WHY???</p>
  <p>The CSV file is about <code>1.51 GB</code> on my PC HDD.</p>
  <p>testing code:</p>
  <pre><code>import polars as pl
  t0 = time.time()
  lazy_df = pl.scan_csv(&quot;kline.csv&quot;)
  df = lazy_df.filter(pl.col('ts') == '2015-01-01').collect().to_pandas()
  print(time.time() - t0)
  
  &gt; Output: 1.8616907596588135
  </code></pre>
  <p>It takes less than <code>2 seconds</code> to scan the whole CSV file, which means that the scan speed is faster than <code>750MB/S</code>. It is much faster than the disk speed, apparently.</p>