Performance regressions (`5.36.0+` and `5.40.0+`)

[techniq]

Describe the bug

In async Svelte versions (5.36.0+) there have been some significant performance regressions (~8x) when rendering many LayerChart charts.

While some subsequent versions have made progress to restore some performance, the more recent releases (5.40.0+) have gotten much worse (~27x).

Below is rendering 20 BarChart instances on different Svelte versions:

version	average	screenshot	REPL
5.35.0 (baseline)	~600ms		REPL
5.39.0	~5300ms		REPL
5.39.13	~4700ms		REPL
5.43.8	~16500ms		REPL

Reproduction

See REPL links above, notable the most recent 5.43.8 version.

For a more real world example, checking out the WIP LayerChart v2 docs PR, which are PR preview deployed here. A good example is going to any component page such as BarChart and clicking "View all" (warning: your browser won't be happy).

I've been using 5.39.13 to development which allowed for async/remote functions support and enough performance to be patient with, but 5.40.0+ can crash the browser (but also fixes some deployment issues in older versions). I recently bumped to the latest 5.43.8 to fix some deployment issues and help report issues.

Logs

System Info

svelte `5.36.0+` and `5.40.0+`

Severity

blocking an upgrade

[gyzerok]

Yesterday I've updated us to 5.43.8 and we've got page freezing reports. There also were some problems with flip animation. So definitely something with each block refactoring isn't backward compatible.

Am going to investigate further and search for something useful. For now can add here that we are not using async.

[dummdidumm]

In prod this shouldn't be this drastic. A quick profile shows that this mainly comes from tracing.js#get_stack, which is done a lot of times in your example. If I return null early in that function the time drops to 1,5 seconds.

That also means that the each block perf regression from @gyzerok might be unrelated.

[MathiasWP]

Yesterday I've updated us to 5.43.8 and we've got page freezing reports. There also were some problems with flip animation. So definitely something with each block refactoring isn't backward compatible.

Am going to investigate further and search for something useful. For now can add here that we are not using async.

@gyzerok we want to begin with performance monitoring of our application - what are you using to get page freezing reports? This sounds very useful!

[techniq]

Thanks so much @dummdidumm 🫶

Tested the REPL example with the newly released 5.43.11 and now seeing ~1000ms 🎉

I'm still getting local lockups within the LayerChart docs even after bump to 5.43.11 which can also peg a CPU core.

I haven't been able to replicate a minimal reproduction in a REPL yet, but I'll keep trying as I can.

If you want to investigate the issue directly, you can checkout the docs-v2 branch (PR 659) in LayerChart and run pnpm install && pnpm dev at the root of the project (or pnpm dev within packages/layerchart and docs individually)

Then navigate to http://localhost:3002/docs/components/BarChart/examples.

Starting at http://localhost:3002/docs/components/BarChart and clicking "View all" will also typically lockup the browser for 1min+ (I usually kill the task)

[Rich-Harris]

Sounds like another manifestation of #17184, which is caused by a linked list of effects somehow turning into an ouroboros

[Rich-Harris]

If that's correct, 5.43.12 should fix it #17191

[techniq]

Thanks @Rich-Harris, there is a very noticeable improvement 🎉

Production build

Production deployments feel relatively close to pre-async (3-4s vs 4-5s for a very demanding page)

Current docs (no async/remote functions)

3-4s

CleanShot.2025-11-19.at.00.27.14.mp4

New docs (async/remote functions)

4-5s

CleanShot.2025-11-19.at.00.21.41.mp4

Development

but development is still pretty painful with async/remote functions.

Current docs (no async/remote functions)

3-4s

CleanShot.2025-11-19.at.00.30.32.mp4

New docs (async/remote functions)

~28s

CleanShot.2025-11-19.at.00.22.55.mp4

---

With that said, development used to take well over a minute (or crash the browser) so there is definitely progress in recent releases.

As always, thanks for all your help!