performance issue: slugify() is O(n^2) in the number of headings on a page, search growth is quadratic #2782

Description

slugify() function keeps the cache of previously created slugs so it can append -1, -2 and soo on to duplicates. To check if any slug has been displayed, it builds an array of every cached key and scans it linearly once per heading:

docsify/src/core/render/slugify.js

Lines 24 to 27 in 030652c

	let count = cache[slug];
	count = Object.keys(cache).includes(slug) ? count + 1 : 0;
	cache[slug] = count;

let count = cache[slug];
count = Object.keys(cache).includes(slug) ? count + 1 : 0;
cache[slug] = count;

Object.keys(cache) assigns a new array of every slug displayed so far and Array.includes() traverse it. For a page with n headings that is n(n+1)/2 string comparisons plus n array allocations totalling n^2/2 strings. Value being searched for is already read on the line above (cache[slug]), so the scan is not needed to obtain it only to distinguish displayed from the ones not displayed.

This runs once per heading during compile and again for every heading of every page while the search plugin builds its index.

Expected behavior

slug generation scales linearly with the number of headings on a page.

Actual behavior

It scales quadratically and because docsify renders markdown in the browser at page load so the cost lands on the main thread on every visit.

I have measured in Chromium with real docsify build, timing from the beforeEach hook to doneEach (markdown compile + DOM render, content supplied by a virtual route so no network latency is involved).

Isolating slugify() itself with the repro below shows where that time goes

16x the headings costs 342x the time; linear would be ~16x and quadratic ~256x.

At 4000 headings, slugify() alone accounts for roughly 2.4 s of the 2.5 s total render

Use Case:
Large single-page API references.

Steps to reproduce

I have created a sandbox here Sandbox
The sandbox loads docsify@5.0.0 from jsDelivr and generates the page content in memory through a virtual route, so nothing but docsify's own render work is being timed no markdown file is fetched.

Steps:

1. Open the sandbox.
2. Click Run all sizes.
3. Read the table: render time against heading count, alongside what linear growth would look like.

Results:

**

16x the headings took 342.4x the time (linear would be ~16x, quadratic ~256x).

**

Environment

- Your OS: Windows 11 Pro (10.0.26200)
- Node.js version: v22.13.1
- npm/yarn version: npm 11.15.0
- Browser version: Chrome on Windows
- Docsify version: 5.0.0 reproduces on the published build (`cdn.jsdelivr.net/npm/docsify@5.0.0/dist/docsify.js`) as well as `develop` @ 030652c. Not present in v4.
- Docsify plugins (if the bug happens when plugins enabled, please try to isolate the issue): none, this reproduces in core and that too with no plugins loaded

Additional Information

• Bug still occurs when all/other plugins are disabled?