Zen Browser is laggy and consumes excessive energy

[phucisstupid]

Captchas

• I have read the instructions.
• I have searched existing issues and avoided creating duplicates.
• I am not filing an enhancement request.
• I have checked that this issue cannot be reproduced on Mozilla Firefox.
• I have checked that this issue can be reproduced once I removed all my Mods and Custom CSS.

What happened?

Platform: macOS 15.5
Hardware: MacBook M1 air, 8GB RAM

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Zen Browser becomes noticeably laggy during regular browsing and consumes an excessive amount of energy, even when only a few tabs are open. This behavior results in high CPU usage and significantly reduced battery life on laptops.

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Steps to Reproduce
1. Launch Zen Browser.
2. Open a few websites (e.g., news sites, social media, or YouTube).
3. Observe system performance over 10–15 minutes.

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Expected Behavior

The browser should remain smooth and responsive during light-to-moderate use, with reasonable energy and CPU consumption.

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Actual Behavior
• The UI becomes sluggish and delayed.
• Activity Monitor / Task Manager shows high CPU usage (often 50%+).
• Energy usage spikes, and battery drains quickly.

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Additional Information
• Issue persists even in incognito mode or with extensions disabled.
• System is up-to-date.
• Other browsers (e.g., Arc, Safari, Chrome) do not exhibit this issue under the same conditions.

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Version

1.12.5b

What platform are you seeing the problem on?

macOS - aarch64

What component is this issue related to?

Other

Relevant log output if applicable

[theoneand33]

#8081 read through that does that help with the laggyness

[wojciech-kulik]

The battery consumption is crazy, before I thought it's my 3-years old MacBook Pro, but now I've just got a new MacBook Pro M4 Pro 14" and the browser drained 20% of my battery in just 1 hour.

Everything is closed, nothing working in the background, I'm just watching YouTube and after 1 hour:

and cpu usage is around 50-70%

[Propheticus]

Disabling any grain effect you might've dialed in on the custom theme colours helps. That one causes quite some load.
Another performance hit comes from rounded corners. There's a "disable rounded corners" zen mod to get rid of those.
Lastly, the advanced pref zen.view.experimental-rounded-view set to false in about:config helps a little.

[wojciech-kulik]

• I don't have any grain set
• zen.view.experimental-rounded-view is set to false by default
• I want rounded corners, I think the latest MacBook should be capable of handling rounded corners :D let's be reasonable

I noticed that the CPU usage is unreasonably high when there is some media - like a video playing on X.com or YouTube. When I stay on Gmail the CPU usage is low, but on X or YT it's 40-60% while for the same website in Safari it's 5-10%.

[Propheticus]

It should be able to handle rounded corners, but Firefox has bugs causing an excessive performance hit when using them.
Until fixed (should be coming in FF v139) the advise is therefore to disable them.

[svyatogor]

I am seeing excessive CPU/Battery usage when playing YouTube videos. Not sure if this is because of Firefox and if Zen can do anything with it but battery drain is really bad. When there a 1080p video playing in a singe tab I see 50% cpu usage on my MBP M3 Pro. Compared do 12% on Chrome and ~4% on Safari.

[franz-dc]

I think this now fixed at 1.12.9b. Scrolling is much smoother on high refresh rate displays and the GPU does not ramp up to full clock speed every time I do something.

I still recommend disabling the rounded corners though as it still lessened the GPU load by half based on my testing.

[phucisstupid]

I think this now fixed at 1.12.9b. Scrolling is much smoother on high refresh rate displays and the GPU does not ramp up to full clock speed every time I do something.

I still recommend disabling the rounded corners though as it still lessened the GPU load by half based on my testing.

not yet, at least on my mac M1, still laggy and overheat

[wojciech-kulik]

Yep, still ~50% CPU usage while playing YT video. (M4 Pro)

[wojciech-kulik]

Unfortunately, Zen is not usable anymore on macOS (at least for me) if you want to keep your battery working for more than 1 year.

Even watching a simple YT video has energy impact between 100-150, while in Chrome it’s 2-10… Another test with scrolling through X.com posts - Zen’s energy impact 1500-2000, Chrome’s 100-150. The battery is basically dying while performing everyday browsing in Zen. Unfortunately, I have to switch to Chrome until this issue is fixed.

Below screenshots taken while watching the same YT video.

[phucisstupid]

This is a genuinely serious issue—I’m not kidding.