The Unseen Efficiency Prices of Trendy CSS-in-JS Libraries

CSS-in-JS is changing into a preferred selection for any new front-end app on the market, as a result of the truth that it provides a greater API for builders to work with. Don’t get me incorrect, I like CSS, however creating a correct CSS structure just isn’t a straightforward activity. Sadly although, in addition to among the nice benefits CSS-in-JS boasts over conventional CSS, it could nonetheless create efficiency points in sure apps. On this article, I’ll try to demystify the high-level methods of the preferred CSS-in-JS libraries, focus on the efficiency points they could introduce occasionally and at last contemplate strategies that we are able to make use of to mitigate them. So, with out additional ado, let’s leap straight in.


In my firm we figured it could be helpful to construct a UI library so as to have the ability to re-use frequent UI items throughout completely different merchandise and I used to be the one to volunteer to get this endeavor began. I selected to make use of a CSS-in-JS answer, since I used to be already actually proud of the styled API that many of the common libraries expose. As I used to be creating it, I wished to be good and have re-usable logic and shared props throughout my parts, so I began composing them. For instance, an would prolong the that in flip implements a easy styled.button. Sadly, the IconButton wanted to have its personal styling, so it was transformed to a styled element alongside the strains of:

const IconButton = styled(BaseButton)`
border-radius: 3px;

As increasingly parts had been added, increasingly compositions came about and it didn’t really feel awkward since React was constructed upon the ideas of this very notion. Every part was high quality till I carried out a Desk. I began noticing that the rendering felt gradual, particularly when the variety of rows acquired greater than 50. Thus, I opened my devtools to try to examine it.

Nicely for sure, the React tree was as huge as Jack’s magical beanstalk. The quantity of Context.Shopper parts was so excessive, that it may simply maintain me up at nights. You see, every time you render a single styled element utilizing styled-components or emotion, aside from the apparent React Part that will get created, a further Context.Shopper is added with a purpose to enable the runtime script (that the majority CSS-in-JS libraries depend on) to correctly handle the generated styling guidelines. This usually shouldn’t be an excessive amount of of an issue, however don’t overlook that parts must have entry to your theme. This interprets to a further Context.Shopper being rendered for every styled factor with a purpose to “learn” the theme from the ThemeProvider element. All in all, if you create a styled element in an app with a theme, three parts get created: the apparent StyledXXX element and two (2) further client parts. Don’t be too scared, React does its work quick and this received’t be an excessive amount of of a difficulty many of the instances, however what if we compose a number of styled parts with a purpose to create a extra advanced element? What if this advanced element is a part of a giant checklist or a desk, the place at the least 100 of these get rendered? That’s when issues come up…


To check CSS-in-JS options I created the best of apps, which simply renders 50 “Hi there World” statements. On the primary experiment, I wrapped the textual content in a standard div factor, whereas on the second, I utilized a styled.div element as an alternative. I additionally added a button that may drive a react re-render on these 50 div parts every time it was clicked. The code for each might be seen on the next gists:

After rendering the element, two completely different React timber acquired rendered. The outputted timber might be seen within the screenshots under:

The React tree utilizing a standard div

The React tree using a styled.div elementThe React tree utilizing a styled.div factor

I then pressured a re-render of the 10 instances with a purpose to collect some metrics with reference to the perf prices that these further Context.Shopper parts convey. The timings of the re-renders in improvement modecan be seen under:

Development render timings for simple div, Average: 2.54msGrowth render timings for easy div. Common: 2.54ms

Development render timings for styled.div. Average: 3.98msGrowth render timings for styled.div . Common: three.98ms

So apparently sufficient, on common, the CSS-in-JS implementation is 56.6% dearer on this instance. Let’s see if issues are completely different in manufacturing mode. The timings of the re-renders in manufacturing mode might be seen under:

Production render timings for simple div, Average 1.06msManufacturing render timings for easy div. Common 1.06ms

Production render timings for styled.div. Average 2.27msManufacturing render timings for styled.div. Common 2.27ms

When manufacturing mode is on, the implementation with the straightforward div appears to profit probably the most by dropping its rendering time by greater than 50% in comparison with a 43%drop on the CSS-in-JS implementation. Nonetheless, the latter takes virtually twice as a lot time to render than the previous. So what precisely is it that makes it slower?

Runtime Evaluation

The apparent reply can be “Erm… you simply stated CSS-in-JS libraries render two Context.Shopper per element”, however in case you actually give it some thought, a context client is nothing greater than accessing a JS variable. Positive, React has to do its work to determine the place to learn the worth from, however that alone doesn’t justify the timings above. The true reply comes from analyzing the rationale why these contexts exist within the first place. You see, most CSS-in-JS libraries depend upon a runtime that helps them dynamically replace the types of a element. These CSS-in-JS libraries don’t create CSS courses at build-time, however as an alternative dynamically generate and replace

When the related React element will get rendered, styled-components:

Parses the styled element’s tagged template’s CSS guidelines.
Generates the brand new CSS class title (or checks whether or not it ought to retain the present one).
Preprocesses the types with stylis.
Injects the preprocessed CSS into the related