5 second delayed intellisense with Svelte Preprocess

[PuruVJ]

Describe the bug
I have a 23 components big app, with TypeScript and SCSS. The intellisense for just about anything(Svelte related or JS related) inside .svelte files takes around 5-6 seconds to show up. In the very beginning when there was just 1 component, it was instantaneous.

This isn't a problem with my computer, because I get instantaneous intellisense in .js, .ts, .tsx file even with 100+ files in the project.

In this svelte app, the intellisense in .ts files is still instantaneous, so it's an issue with the Svelte extension only.

To Reproduce
Steps to reproduce the behavior:

1. Clone this repo: https://github.com/puruvj/macos-web
2. Run npm install
3. Go to any Svelte file with some local state and try typing out variable names in the <script> tag or in the template or the snippets autocompletion of if, each, await blocks, etc. See the speed of intellisense
4. Now, delete node_modules, go to the same .svelte file as before, and now try intellisense. It will be instantaneous now.

I think this issue is happening because of svelte-preprocess, because I have noticed this lag in every other Svelte project with TS/SCSS with svelte-preprocess, whereas projects without it are really fast.

Expected behavior
I want the intellisense to be as instantaneous with svelte-preprocess as it is is without it 🙂

System (please complete the following information):

• OS: Windows
• IDE: VSCode
• Plugin/Package: Official Svelte extension for VSCode

[dummdidumm]

The intellisense gets slower for me as well, which is expected since there are now much more files the TypeScript language service has to scan. I don't see the massive slowdown you see however. Could you give more details?

• which file exactly should I open, what exactly should I type to see it?
• what exactly to you mean by "intellisense"? Auto completion suggestions?
• Which version of the extension do you have installed
• What's your hardware spec (roughly)?

[PuruVJ]

Sure!

I noticed an additional thing that when I open the workspace in VSCode and open a .svelte file, the lag isn't that bad. It's like 1-2 seconds, which is manageable, but as I open more files and work on them, then it starts getting slower. Closing the files doesn't make it any faster.

And here are the details for your questions:

• Open components/Dock/Dock.svelte, and type anywhere inside the template an if block, then put in the mouseX variable in the condition and notice autocomplete speed. or if you're in the each block, create an if block inside it and try typing any variable avilable in that scope
• Autocomplete only, my bad 😅
• v105.3.4
• i5 8th gen processor, SSD, 8 GB RAM Windows 11 laptop

[dummdidumm]

I tried to narrow this down, but no luck. The biggest delay I get is within a function of the TypeScript language service which we have no control over. I'm not sure if this is what slows this down for you. What you could try to narrow this down:

Add this to your user settings (JSON):

"svelte.plugin.css.enable": false,
"svelte.plugin.html.enable": false,
"svelte.plugin.svelte.enable": false

This should only keep the TS/JS intellisense of the extension enabled. Is this noticably faster then? If yes, by how much?
If it isn't, or it is a little faster but not that much, then you can go to the user settings and try to disable all language features except svelte.plugin.typescript.completions.enable. This should give you a good idea then of how much of the slowdown is directly related to the completions code path and not to some other synchronous code being executed and blocking the completion execution.
List of all settings (also found via VS Code settings UI): https://github.com/sveltejs/language-tools/tree/master/packages/language-server#list-of-settings

[PuruVJ]

Hi, the delay has reduced to 2-3 seconds after I modified the settings json with your code. Which is manageable, but I rely on these features a lot, can't disable them 🥲

Thank you for your time

[dummdidumm]

Disabling the settings was meant to narrow down where the slowdown comes from, not as a solution. If you reenable them one by one, which one does the have the most impact (my guess is svelte.plugin.svelte.enable)?

[PuruVJ]

Ah ok, I'll try that

[dummdidumm]

The latest release includes some performance improvements, completions should have higher priority now. Please let me know if it improved for you.
Also, could you add "svelte.trace.server": "messages" to your user settings JSON, then reproduce your slowness for 1-2 minutes and then copy the output of the Svelte extension in here? I'm interested in the response times that are logged for the various requests.

[PuruVJ]

Where do I see the output? 😅

[dummdidumm]

At the bottom where the terminal is, there's a tab called "Output". Select it, then select "Svelte" from the dropdown to the right.