zlacker

>>mconle+(OP)
TL;DR: Windows Defender had a bug that made certain system calls expensive on CPU cycles when Defender's Real-time Protection feature is enabled. After discovery, Mozilla reported this issue to Microsoft. Microsoft is releasing a patch that should result in lower CPU usage when using Firefox on sites like YouTube (a ~75% CPU usage reduction was noted when browsing YouTube in Firefox with the fixed version of Defender).

It seems like the HN submission form truncated the # from the end of the URL I linked to, which linked to the relevant comment. I'll try that here:

https://bugzilla.mozilla.org/show_bug.cgi?id=1441918#c82

and

https://bugzilla.mozilla.org/show_bug.cgi?id=1441918#c91

>>mconle+h3
Well, also Firefox is making an excessive number of calls to that slow system call compared to other browsers (Chrome, Edge).

>>Diggse+Ad
Well, it was fast system call until MS added AV hook to it.

>>zokier+gx
My comment was only intended to add missing information to the TLDR (since this fact is important in the linked thread) not to say that Firefox is at fault.

Now that you raised it however, even if the system call used to be fast, Firefox is making an extremely high number of calls to that sytem call, and there's always going to be some overhead to that. There are almost certainly ways that Firefox could reduce the number of calls it needs to make.