That could be almost everything on the route from your router to the server what you playing.
Check traceroute for hop latences.

https://help.elderscrollsonline.com/app/answers/detail/a_id/30903/~/how-do-i-do-a-pathping-and-tracert-to-the-servers-to-test-my-connection?

Ping issues are mostly traffic problem not game itself. Your ISP can have some problems too, some server on the middle of the data way to the game server can have problems and can be affected only eso because other things will have different "way".

Btw your speed test only showing your speed to the nearest server, so it shows only that your own connection is good. Others games will have different route to theirs game servers.

Hi, google should be fine, I used it for a while too.

Anyway high ping is weird if it affect only one online game. I dont remember exactly how much hops I have but i think that is about 13-15 for EU (more with higher katency on some hops for NA). For EU server and have ping about 60. On the NA have something between 120-300, spikes are up to 500 but most of the time is good about 120.

I can remember two weird issues with ESO what i had before...
First was weird disconnects from the game. Could be even 5 in one hour. For no reason.
Then i just updated all drivers (mobo, network, even gpu) and disconnects was gone.

Second was interesting. When me and wife tried to parse dummy or go to pvp in the same time.
She was at one house alone with parse dummy, me was at my house at dummy, she started parse it was good and when I started parse after while, both of us get weird lags. So i changed router and problems was gone.

I dont know on what server you playing, you can try ask in the zone chat when you will have high ping if anyone have similar or same problems.

I am really sorry that i can't help.