Anyone noticed Chrome slowdown/lag sice Avast 25.5?

[chris...]

The web shield can be configured: certain elements such as “QUIC/HTTP3 scanning” can be disabled without completely disabling the shield.

Try unchecking these options and see if that helps. If the problem no longer occurs, check them back in one by one until you find the one that is causing the issue.

This will not solve your problem, but it may give avast an indication.

[chris...]

It would appear that the “QUIC/HTTP3-Scanning” setting is indeed the source of the slowdowns (with W11?).

"QUIC/HTTP3 scanning" slows down the loading of some websites

Hello! Since about two weeks, loading some websites on a new PC (AMD Ryzen 9 9900X, Windows 11 Pro + AVAST free - always up to date) takes extremely long. No changes have been made to the settings of AVAST free so far. However, if you disable URL-Guard => QUIC/HTTP3-Scanning the pages load normally. On another, older PC (with the same AVAST settings), the affected websites load quickly - even though QUIC… is enabled How do I find the cause? Please consider that I am not an expert, so I need a…

If you confirm this, it’s a minor inconvenience while waiting for avast to look into it, as the web shield should continue to protect you for the most part.

[FrauHolle]

I forgot to mention in my thread that Windows 10 Pro is installed too on the old PC. On that PC, QUIC/HTTP3-scanning works without any issues - it’s only on the PC with Windows 11 Pro that some pages fail to load.

Is anyone else having problems due to QUIC/HTTP3-Scanning?

PS: Do the AVAST devolopers actually read the forum? Are they even aware of this issue? I’m asking because this problem has been around for quite some time.

[chris...]

Anyone noticed Chrome slowdown/lag sice Avast 25.5?

Thank you @TheOwner for the videos and the additional info! Can you please record this experience using the “Windows Performance Analyzer” tool? Once that’s done, please attach the log via the Support Tool: A) Capture the performance log: Launch the elevated command line (cmd.exe as Administrator) Change the log destination by running the command: cd c:/ Start recording via command wpr -start "C:\ProgramData\Avast Software\Avast\profile.wprp" Reproduce the performance issue (e.g. switch to C…

Anyone noticed Chrome slowdown/lag sice Avast 25.5?

Hello @TheOwner and @blade9722 , thank you very much for sending the logs! We currently suspect this problem could be related to the “Script Shield” feature. Can you please check if the problem persists when you disable the option “Web Shield: Protect Google Chrome with Script Shield“ in Avast Geek:Area? This article explains how to access the Geek Area: How to use the Avast Geek settings area in Avast Antivirus | Avast In the “Web Shield” section of the same article, you can see more details…

[FrauHolle]

File ID: SJTJI

[FrauHolle]

The same here, just like the other users. I disabled all Script Shields in the Geek:Area but after reboot there’s no change - both with Firefox and Edge. The only thing that helps is disabling “QUIC-HTTP3-Scanning”.

[blade9722]

Hello, I’m back.

I confirm that QUIC/HTTP3-scanning is the cause of the issue (which is not a slowdown, it’s causing web pages not downloading at all) in Windows 11. Windows 10 seems unaffected.

Another, for me known from years setting that sould be disabled is HTTPS scanning. This is causing a lot of slowdowns in web browsing, even if I think it doesn’t stuck the download of pages.

[prokopes_]

Hi, last week (05/Nov/2025), we released a Virus Definitions update that should address this problem. This update is applied automatically, but it only becomes effective after the browser is restarted. Can you please check if the problem persists today, and if so, send us new logs with the performance recording as per the steps above?

Two notes:

• There seems to be multiple culprits and specific factors - various users are reporting different behaviours. This update is focused on Script Shield, but it may not fix the problem for everyone (Script Shield does affect even other components, but it may not fix it for you).
• We realize that the performance logs we requested are not easy to obtain and we respect if you don’t wish to do all this work. It is also helpful if you simply report whether the problem is fixed or not (although logs are always appreciated).

Thank you all for your help and patience.

[chris...]

hello,
Is there a link between the script shield and QUIC/HTTP3 analysis?

Because disabling it seems to solve the problem more effectively than disabling the script shield or even applying the November 5 VPS update.

[FrauHolle]

As the updates are installed automatically and the log file was created two days after release, it should still be up to date (file ID: SJTJI). But unfortunately the update did not work.

[prokopes_]

Thank you @FrauHolle , we will check these logs. I should’ve mentioned that our fix was aimed at Chrome browser. (we are checking the performance in other browsers too but Chrome was the focus here)

[prokopes_]

@FrauHolle we checked the logs, and this appears to be a different issue than the Chrome one. Can you please send us a new, different set of logs? Again, the steps are quite complicated, and we respect if you don’t wish to spend time on this, but they would help us understand the behavior and move our investigation forward.

A) Generate logs:

1. Disable the Avast Self-Defense module in - Avast - Menu - Settings - Troubleshooting - uncheck “Enable Self-Defense”.
2. In the same menu, scroll down to the bottom and check the option “Enable debug logging“.
3. Download and import the attached registry file - double-click it and confirm import of the required keys (they allows debug logs to exceed the default 8 MB file size).
4. Ensure that the QUIC-HTTP3-Scanning” is enabled in Web Guard settings.
5. Restart your computer and wait till Avast is fully loaded.
6. Open Firefox or Edge and browse a couple of websites - reproduce the poor performance (for around 5 minutes, if possible).

B) Submit logs:

1. Follow the Support File submission process here: http://support.avast.com/en-ww/article/submit-support-file/
2. In the field “Ticket number”, you can use the ID of this thread: 888995
3. Finish the support file creation (no need to attach any file).
4. Once the support file is created, please share its “File ID” in your response to this message.

C) Clean up:

1. (Delete the log file “StreamFilter.log“ - it could grow to a couple of hundreds of MB. You can find it in this directory: C:\ProgramData\Avast Software\Avast\log)
2. Enable the Avast Self-Defense module in - Menu - Settings - Troubleshooting - check “Enable Self-Defense”.
3. In the same menu, scroll down to the bottom and uncheck the option “Enable debug logging“.

Let me know if you have questions.

Avast.StreamFilter.FileLogSize-2GB.reg (538 Bytes)

[FrauHolle]

No Problem! I’m using the free-version. That’s why I like to take the time for it.

File-ID: 1LMYM

[Dinobot2]

@prokopes_

Hi, last week (05/Nov/2025), we released a Virus Definitions update that should address this problem. This update is applied automatically, but it only becomes effective after the browser is restarted. Can you please check if the problem persists today, and if so, send us new logs with the performance recording as per the steps above?

Two notes:

• There seems to be multiple culprits and specific factors - various users are reporting different behaviours. This update is focused on Script Shield, but it may not fix the problem for everyone (Script Shield does affect even other components, but it may not fix it for you).
• We realize that the performance logs we requested are not easy to obtain and we respect if you don’t wish to do all this work. It is also helpful if you simply report whether the problem is fixed or not (although logs are always appreciated).

Thank you all for your help and patience.

Can anyone, maybe multiple anyones, confirm if this indeed fixed the problem?

[dlt]

Hi,

No, the issue is still there. It is related to HTTP3 requests only and seems to affect most browsers. Some browsers decide to fall back to HTTP2 if QUIC does not work and it may look like everything is fine, but when HTTP3 remains in use, most of the content either does not load at all or loads after several seconds. I was able to replicate this with Firefox with latest Avast (11/2025).

debuglog VHSV1.

By the way, I can replicate this with our own servers which support HTTP3/QUIC, so the issue is definitely with HTTP3 scanning mode. Once disabled, everything works perfectly. We started to get complaints from our customers who use our servers that their content does not load when Avast is in use, and in all cases problem goes away when we ask them to disable H3 scanning…

[TheOwner]

Hello,

regardless of whether the shields are enabled or disabled, the issue continues to persist. I am submitting new logs with two attachments: one collected with the shields enabled and the other with all shields disabled, in which the issue is reproduced. In both cases, however, Chrome is equally slow.

File ID:YX4DB