For styles' CSS maybe look at adding CSS Containment property support http://news.softpedia.com/news/chro...ment-and-performance-measurement-506482.shtml

New CSS contain property

The team didn't stray much from their plan, and now Chrome features support for CSS containment, via the CSS contain property, which prevents child elements from showing up outside the boundaries of their parent element.

A good reason for developers to implement CSS containment on their websites is to speed up page load times. Google engineers played around and detailed the advantages of using the contain property in a blog post in June.

CSS contain support is only available in Chrome 52 and Opera 40 (alpha stage). Firefox devs have shown public interest in integrating the property into their browser, but no code to support it has landed in the browser until now.
cheers Russ from the first link https://justmarkup.com/log/2016/04/css-containment/ for off canvas css as i understand it would be something like the following for FA+ i suppose

@media (max-width: 800px)
.xbOffCanvasContainer.marginLeft {
    overflow-y: auto;
    position: relative;
    transition: 300ms ease all;
    margin-left: 250px;
    contain: paint;

@media (max-width: 800px)
.xbOffCanvasContainer {
    overflow: hidden;
    transition: 300ms ease all;
    width: 100%;
    contain: paint;
no idea just stabbing in the dark heh

If you build a off-screen navigation or similar, the browser paints the content completely although it is not visible on load. By setting contain: paint; the user agent can skip the paint off the off-screen element and therefore paint all the other content faster.

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