[Performance Issue] On websites where Dark Reader is DISABLED, experiencing extremely bad performance for page loads

Start by reproducing the listed steps with “detect dark theme” enabled and Dark Reader disabled for GitHub, then inspect the attached performance trace. Compare behavior in Chrome and Firefox; done means disabled sites no longer show broken rendering or delays of up to 25 seconds after opening a new tab.

Prerequisites

• I searched for any existing report about this performance issue to avoid opening a duplicate.
• I have restarted the web browser to check if that resolved this performance issue.
• I can reproduce this performance issue in a new, unmodified web browser profile with Dark Reader installed as the only extension.

Performance Issue Description

See the attached video. Recently, when opening a new tab, Dark Reader has been causing websites to appear broken for as long as 25 seconds. Worse yet -- this happens even for websites where Dark Reader is disabled! The issue resolves itself when clicking the Dark Reader icon, or when Dark Reader is disabled. See the attached video. Replicated using a new blank Chrome profile. The issue only happens when "detect dark theme" is enabled. I was able to reproduce in Firefox 156.0 as well as Chrome.

Website Address

https://github.com/darkreader/darkreader

Steps To Reproduce

1. Install Dark Reader
2. Go to GitHub and disable Dark Reader for github.com
3. Disable and re-enable Dark Reader in Extensions settings (the issue reproduces ~20% of the time without doing this step, and maybe ~90% of the time when including this step!)
4. Open a new tab
5. Paste the URL (https://github.com/darkreader/darkreader) and press return

Performance Profile

Trace-20260918T015019.json.gz

Screenshots

Video demonstrating the issue:

https://github.com/user-attachments/assets/8e8e77a7-692f-462d-a79e-257001e0f3fb

Video of my screen while I recorded the attached performance trace:

https://github.com/user-attachments/assets/2ada7166-329e-4d3f-8bed-a2240e3e7df2

Operating System

macOS 27.0

Web Browser name and version

Google Chrome Version 153.0.8010.53 (Official Build) (arm64)

Dark Reader version

4.9.132

Does this happen when Dark Reader is disabled?

No

Does this happen in other web browsers?

Yes

Additional Context

• I understand that "detect dark theme" is a computationally intensive thing to do. I would get a second or two, but 25 seconds is far too long for something like this and it makes browsing the web frustrating since this happens seemingly at random.
• This is not simply Dark Reader's "dark theme" version of GitHub; it's a broken, half-rendered version. For example, even with Dark Reader fully enabled, the "Code" button should be green, but in this broken state it is black, and there are other visual artifacts as well.