Why is the `sx` prop so much slower? - reactjs

Question

As per MUI's own doco, and this answer - components using sx render significantly slower than components using other styling mechanisms.
On the surface, it looks like sx is just an alternate convenience API for doing the same thing - so I wouldn't expect it to have such a different performance profile.
My question is: Why is the rendering of a component using sx so much slower - what's it doing so differently? Is it a whole different styling engine or something?
I'm curious about the possibility of optimising it, or coming up with a compromise that retains most of the usability but omits whatever feature is causing the slowdown.
Please note, this question is about "why is the performance so different" - not "why do you think the difference doesn't matter".

Answer 1

As I started to dig into this, I realized that I needed to measure the performance of different scenarios in order to have any confidence in my understanding of the performance aspects of the sx prop.
I believe that the performance information in the MUI documentation was gathered using some variation of this repository: https://github.com/mnajdova/react-native-web. The react-native-web repo was used as a starting point because of its "benchmarks" package which contains a useful framework for measuring the performance of different React element rendering/styling approaches.
I created my own version here: https://github.com/ryancogswell/mui-style-benchmarks. You can use this as a starting point to dig into this further. Below are the measurements I made and my conclusions.
My Results for the "Mount deep tree" Benchmark
This test renders 639 elements with approximately 17 CSS properties each except for the cases ("..._minimal", "..._medium")
which reduce the number of CSS properties to show the performance impact.
Styling Implementation
Time in ms
Implementation Desc
inline-styles
22.78
No styling engine, just use style prop
mui_sx_full
36.89
MUI Box sx prop with 17 CSS properties
mui_sx_medium
24.09
MUI Box sx prop with 9 CSS properties
mui_sx_minimal
18.15
MUI Box sx prop with 4 CSS properties
mui_styled_box
22.38
MUI styled MUI Box with 17 CSS properties
mui_styled_box_minimal
17.90
MUI styled MUI Box with 4 CSS properties
tss_react_makestyles
17.10
makeStyles from tss-react with 17 CSS properties
mui_styled
16.93
MUI styled div with 17 CSS properties
mui_styled_minimal
13.77
MUI styled div with 4 CSS properties
emotion_styled
16.69
Emotion styled div with 17 CSS properties
emotion_styled_minimal
12.76
Emotion styled div with 4 CSS properties
emotion_css
12.58
Emotion css div with 17 CSS properties
Conclusions
MUI styled (e.g. import {styled} from '#mui/material/styles') only adds a small amount of overhead
to Emotion's styled.
tss-react performs similarly to MUI styled.
Emotion styled, Emotion css, MUI styled, and the MUI sx prop are all more expensive when there are
more CSS properties passed to the styling engine.
The performance of the sx prop degrades more quickly than the styled API as more
CSS properties are passed to it. With 17 CSS properties the performance is much worse than the styled API (2x).
The sx prop performs just fine for a small number (e.g. < 5) of CSS properties. Particularly, if you
are already using a MUI component in a given situation, there is no meaningful performance difference
between wrapping it with styled or using the sx prop if you are just using a small number
of CSS properties.
What is the cause of the sx prop slowness?
Is it a whole different styling engine or something?
It is not a different styling engine. The output of the work done for the sx prop is fed into the styled API of the main styling engine (e.g. Emotion or styled-components); so using the sx prop with the Box component is guaranteed to be slower than the equivalent styles using styled on a div because the sx prop still uses styled in the end but does additional work first.
What is the additional work done by the sx prop?
The MUI styled API calls styleFunctionSx in order to transform the CSS properties in the sx prop to the form expected by the styling engine.
styleFunctionSx traverses all the CSS properties in the sx prop
For each CSS property, it checks to see if it has a style function for transforming the prop value. These style functions provide support for shorthand notation for some properties such as padding and margin and support for using theme spacing units instead of pixel values.
Each of the style functions leverages the style function in https://github.com/mui/material-ui/blob/v5.5.3/packages/mui-system/src/style.js to handle some common transformations of prop values including the support for breakpoint-specific values.
The net effect is that for each CSS property there are a number of lookups and function calls to see if the CSS property needs to be transformed even in the cases where the value passes through without changes.
I'm curious about the possibility of optimising it, or coming up with a compromise that retains most of the usability but omits whatever feature is causing the slowdown.
I'm sure that performance improvements are possible for the sx prop, but I don't think there is any single silver bullet for easily making it faster. Instead it will probably require a large number of little changes that are each barely measurable, but cumulatively provide decent improvement. The challenge is to make those changes without simultaneously making the code more complex and/or harder to maintain and/or more error prone.
The main compromise that "retains most of the usability" is to use Emotion's css prop directly. It can be used directly on elements in a similar fashion as the sx prop -- you just lose the shorthand notations and theme lookups that the sx prop provides. The theme lookups (e.g. for colors or spacing units) are easy to get directly from the theme by using the useTheme hook in the component. The theme.breakpoints API can be used instead of the breakpoint shorthands; though the sx breakpoint features are much nicer from a DX standpoint.