There are a ton of unknowns here, it may end up being as easy as disabling some game setting related to network optimization or ipv6 use. Most of troubleshooting is just thinking of more questions to ask!
Are you launching the game using wine/proton/???
What traceroute and mtr have told us is that ICMP echo requests (called a ping) don’t get there reliably, but that can happen for multiple reasons. Maybe a server along the path doesn’t feel like passing your ping along and drops it. There’s a point in their machinery which blizzard doesn’t want you peeking past, and other companies might have their own reasons even if the route is healthy.
A friend says the game has two network sockets that have individual pings (latency measure this time not echo) and you can see them by hovering your mouse over the indicator. Which is it that’s misbehaving?
If you can traceroute to some unrelated cloudflare dns server like 1.1.1.1, then the problem seems likely to be outside your machine, outside your home network, and then we scrutinize what hop it’s failing at to see if we can rule things out further. (I’m making assumptions obviously, this is just how I’d think about it.)
Maybe that’s your ISP, or something in a data center with a hostname related to wow itself?
SteamOS is an immutable OS so many changes will be overwritten at your next update, but you can always pull a tool down temporarily, or keep a script saved on your desktop!
Pop the hood open
sudo steamos-readonly disable
Pacman is a package manager you’ll have on steamos/arch. Refresh pacman keys and resync database.
sudo pacman-key --init
sudo pacman-key --populate
sudo pacman -Syy
Install the tool you want (traceroute) sudo pacman -S traceroute
Close it up again until your next change
sudo steamos-readonly enable
Once you’re in a position to use traceroute you can just follow it with an IP or a domain.
traceroute 1.1.1.1
It will check each step (hop) along the networking path path between you and that target with details like the round trip time a message took to go there and back to you. Look for a hop where the latency gets bad and stays that way for the rest if the hops like a clog in a pipe. If you don’t see that you’ll probably want to examine with some other tools like mtr that go into more depth.
https://man.archlinux.org/man/mtr.8.en
I had a lag spike situation on a steamos machine playing a game called shape of dreams when using wifi but not ethernet, and it wanted a network card driver update. Kind of a shot in the dark but you might try it.
I’m reading that there’s a network stats API that addons can use https://warcraft.wiki.gg/wiki/API:GetNetStats
Maybe there’s a network performance monitor addon you could find, but I don’t have suggestions there.