In the new idle premium, it mentions there is an idle standalone client for Windows. I bought premium, but couldn't find any download links. Where is it?

https://www.warzone.com/wiki/Standalone_Client

Right, missed that there's an idle option. The latest blog post said access to an idle client, and I assumed there would be a new one.

This looks like a port of the mobile app; performance wise it seems worse than using a browser.

Its the same application compiled for different platforms.

Speed wise I always found that some things work better in browser and whatever became UJS works better on the apps.
But it is taking up significantly more resources (cpu / memory) than a browser does.

the standalone client (and classic app) contain the full game. idle app is its own mobile app for some reason.

Yeah I never liked the idle-only app. I.i.r.c. it was added for marketing sake?

Especially because it got the full game build into it but just hidden:
• SP for the tutorial
• MP for Arena/CW
• Community for the forums/clans/chatroom