# Does PLT correlate with Speed Index

**URL:** <https://discuss.httparchive.org/t/does-plt-correlate-with-speed-index/286>\
**Category:** Analysis\
**Created:** [June 18, 2014, 7:01pm UTC](https://discuss.httparchive.org/t/does-plt-correlate-with-speed-index/286 "2014-06-18T19:01:19Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![rcs](https://avatars.discourse-cdn.com/v4/letter/r/d2c977/32.png) [@rcs](https://discuss.httparchive.org/u/rcs)\
**Post date:** [June 18, 2014, 7:01pm UTC](https://discuss.httparchive.org/t/does-plt-correlate-with-speed-index/286/1 "2014-06-18T19:01:19Z")

</div>

Others have [argued](http://www.stevesouders.com/blog/2013/05/13/moving-beyond-window-onload/) that PLT is a poor metric for web latency.

I was curious: how well do SpeedIndex values correlate with PLT values? The strength of their correlation would give us a sense of exactly how bad PLT is as a metric (assuming that SpeedIndex is what we really care about).

To answer my question I looked at the proportion of PLT to SpeedIndex (for mobile sites):

```
SELECT
  url,
  onLoad / SpeedIndex on_load_to_si
FROM
  httparchive:runs.2014_05_15_pages_mobile
WHERE
  onLoad IS NOT NULL
  AND SpeedIndex IS NOT NULL
ORDER BY
  on_load_to_si

```

And the result:

![](https://yyz1.discourse-cdn.com/flex035/uploads/httparchive/296/6ce7966c20155a39.jpg)

It looks like the correlation is pretty close for most sites. About 15% of the httparchive sites have a better SpeedIndex than PLT relative to the norm, and only a few percent of sites have relatively better PLTs.