The final test (https://benfrain.com/selector-test/3-01.html) I ran was to hit the page with a bunch of expensive properties and values. Consider this rule:

.link {
    background-color: red;
    border-radius: 5px;
    padding: 3px;
    box-shadow: 0 5px 5px #000;
    -webkit-transform: rotate(10deg);
    -moz-transform: rotate(10deg);
    -ms-transform: rotate(10deg);
    transform: rotate(10deg);
    display: block;
}

With that rule applied, here are the results:

Here all browsers are at least up with their slowest selector speed (IE was 1.5X slower than its slowest selector test (10) and the Android device was 1.3X slower than the slowest selector test (test 6)) but that's not even the full picture. Try and scroll that page! Repaint on those kind of styles can bring a browser to its knees (or whatever the equivalent of knees is for a browser).

The properties we stick inside the braces are what really affects performance. It stands to reason that scrolling a page that requires endless expensive re-paints and layout changes is going to put a strain on the device. Nice HiDPI screen? It will be even worse as the CPU/GPU strains to get everything re-painted to screen in under 16 ms.

With the expensive styles test, on the 15" Retina MacBook Pro I tested on, the paint time shown in continuous paint mode in Chrome never dropped below 280 ms (and remember, we are aiming for sub–16 ms). To put that in perspective for you, the first selector test page, never went above 2.5 ms. That wasn't a typo. Those properties created a 112X increase in paint time. Holy expensive properties Batman! Indeed Robin. Indeed.