1440846 - Firefox uses 60% of my CPU while downloading a file

Status: UNCONFIRMED

(Firefox :: Downloads Panel, defect, P3)

Description

[Cameron Simpson]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:59.0) Gecko/20100101 Firefox/59.0
Build ID: 20180219114835

Steps to reproduce:

Visit the Calibre download page. Download the latest Calibre DMG for MacOSX. 

https://calibre-ebook.com/download


Actual results:

Observe the CPU spike in my toolbar widget, pop up Activity Monitor, see firefox with 60% of the CPU. Pop up FF's download manager (previously closed). Watch the download finish and the CPU instantly revert to essentially idle.


Expected results:

It should take almost no CPU to maintain a progress bar, and I can't imagine what else is going on. I do have a bunch of extensions, none seems to have any interest in download activity.

Comment 1

[Cameron Simpson]

Some additonal runs suggest this is highly correlated with the amount of progress bar updates (or download chunks received). I have the pleasure of being on a particularly dodgy network right now, which means I got to observe the download vary in speed wildly. When stalled CPU usage was very low, when good CPU usage was very high.

This firefox is running through a local squid proxy, so it is not any kind of direct network interaction.

The download I'm using is about 75MB. The CPU usage exists whether the download manager is visible or not.