Zen GPU Helper intermittently consumes ~700% CPU on macOS

### Preliminary Checks - [x] I have read and understood the important section above. - [x] I have searched existing issues and avoided creating duplicates. - [x] I am not filing an enhancement request. - [x] I have checked that this issue cannot be reproduced on Mozilla Firefox. - [x] I have checked that this issue can be reproduced once I removed all my Mods and Custom CSS. ### What happened? Zen Browser is showing unusually high CPU usage from the Zen GPU Helper process. In the attached screenshot: Zen GPU Helper: approximately 689% CPU Total CPU activity: 81.5% System load: - 1 minute: 8.33 - 5 minutes: 10.75 - 15 minutes: 8.93 The system has 10 logical cores; it is the MacBook Pro M5 This does not appear to be normal browser load. The GPU Helper process is consuming the equivalent of almost seven CPU cores and causes high system usage, increased power consumption, heat, and potentially reduced battery life No mods are being installed with Zen ### Expected behavior Zen Browser and its GPU Helper process should return to low CPU usage when there is no graphics-intensive activity, video playback, animation, or other demanding browser workload ### Actual behavior The Zen GPU Helper process continues consuming more then 500% CPU, sometimes up tp 900%, even though there is no obvious workload that should require this level of processing. Closing or restarting Zen Browser may temporarily stop the issue, but the behaviour can return. ### Steps to reproduce 1. Open Zen Browser. 2. Use the browser normally with several tabs open. 3. Leave the browser open for some time. 4. Check CPU usage in a system-monitoring application. 5. Observe that Zen GPU Helper may begin consuming several hundred percent CPU. ### Screenshots and videos <img width="413" height="647" alt="Image" src="https://github.com/user-attachments/assets/effe9916-5953-41fe-ab86-e269f79aa8c4" /> ### Version 1.21.9b (64-bit) ### What platform are you seeing the problem on? macOS - aarch64 ### What component is this issue related to? Performance ### Relevant log output if applicable ```shell ```