[core] perf: sx

[romgrk]

Improve sx performance by removing allocations.

Example user code:

Before: 23.4ms +-1.6
After:  15.2ms +-1.1

n=100

We get roughly 35% less rendering time for the example above which makes heavy use of the sx prop, so I guess user-code would see something in the 0-40% less rendering time depending on how much they use that prop.

We can still get a lot more improvement by eliminating more memory allocations (I think we can get that example to at least 50% less rendering time), but the remaining changes would require more substantial work. The format of the style({ [prop]: value, theme }) sx style handlers is expensive, we could instead use something like style(prop, value, theme), though IIUC the system props also use those so there's a bit of refactoring to do.

The logic to create an empty breakpoint object for each sx object/subobject is also expensive, I've tried to remove it by lazily initializing the breakpoints but handleBreakpoints is used too much in the codebase so that's another large refactor.

[mui-bot]

Netlify deploy preview

https://deploy-preview-44254--material-ui.netlify.app/

@material-ui/system: parsed: +1.44% , gzip: +1.57%
PigmentHidden: parsed: +1.91% , gzip: +1.90%
Slide: parsed: +1.81% , gzip: +1.95%
Fade: parsed: +1.89% , gzip: +1.89%
Zoom: parsed: +1.89% , gzip: +1.89%
Box: parsed: +1.59% , gzip: +1.87%
Grow: parsed: +1.83% , gzip: +1.93%
@material-ui/lab: parsed: +0.36% , gzip: +0.52%
@material-ui/core/styles/createTheme: parsed: +2.60% , gzip: +2.90%
SpeedDialAction: parsed: +0.78% , gzip: +0.95%
ListItemText: parsed: +1.45% , gzip: +1.50%
TextField: parsed: +0.67% , gzip: +0.86%
CardHeader: parsed: +1.44% , gzip: +1.48%
Autocomplete: parsed: +0.67% , gzip: +0.82%
FilledInput: parsed: +1.15% , gzip: +1.42%
Fab: parsed: +1.16% , gzip: +1.37%
createBox: parsed: +2.71% , gzip: +2.97%
@material-ui/core: parsed: +0.20% , gzip: +0.26%
LoadingButton: parsed: +1.03% , gzip: +1.26%
@mui/joy/Box: parsed: +1.78% , gzip: +2.09%
and 191 more changes

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Generated by 🚫 dangerJS against 3455a04

[romgrk]

@mui/utils/deepmerge is incredibly unefficient but it's used all across the codebase and I don't have enough bandwidth to try to replace it. But it should be replaced.

[romgrk]

I would have expected to have to add .internal_cache to the system's createTheme function only, but I ended up needing to add it at various locations. It would be neat to pass all our themes through that function to avoid repeating internal details in various places.