Google Chrome	54.0.2793.0 (Official Build) canary (64-bit)
Revision	6667812b2dffe3f6bf794b2b040504e5a3ad7cb7-refs/heads/master@{#404564}

See the attached listings of the selectors being executed on mint.com's transaction table and zillow's detail page. The mint page requires a login but zillow does not.

To see the flood of selectors and also look at the time this snippet in the console:

$qsa = Document.prototype.querySelectorAll;
$qsae = Element.prototype.querySelectorAll;
$totalTime = 0;
function $qsaLog(s) { console.log(s); var t = performance.now(); var r = $qsa.call(this, s); $totalTime += (performance.now() - t); return r; }
Document.prototype.querySelectorAll = $qsaLog;
Element.prototype.querySelectorAll = $qsaLog;

Then after interacting with the page log $totalTime to see the time spent in querySelectorAll.

Mint is from: https://wwws.mint.com/transaction.event

1) Click any transaction category.

This runs querySelectorAll for 300ms on my Reks Chromebook.

Zillow is from: 
http://www.zillow.com/homes/for_sale/San-Francisco-CA/house,condo,apartment_duplex,townhouse_type/15074242_zpid/20330_rid/2-_beds/0-2000000_price/0-6976_mp/1500-_size/elevator_att/globalrelevanceex_sort/37.799917,-122.386794,37.777533,-122.459235_rect/13_zm/0_mmm/

1) Click anywhere in the text of the page.

or

1) Click the little calculator icon.
2) Click "30 year fixed" drop down.

Clicking the dropdown in (2) on a Reks Chromebook takes runs querySelectorAll for 800ms.

If you look at the sampling of the selectors they all end with [id="something"] and would be much faster if we instead used the getElementById fast path (or the websites weren't doing something so crazy, YUI doesn't seem to be helping here either).
 

Deleted: mint-qsa-selectors.txt 10.9 KB

mint-qsa-selectors.txt 10.9 KB View Download

Deleted: zillow-qsa-selectors.txt 86.7 KB

zillow-qsa-selectors.txt 86.7 KB View Download