Why is the new AI assistance so slow?

Previously, I used to be able to pressed Ctrl + \ to get an AI Assistant prompt window that would work on the code in the current file. It was fast and allowed me to do follow-up prompts and apply or discard changes. Now the prompt delegates to the Junie AI Agent and it takes significantly longer to suggest changes. I also can't seem to change which agent does the work; it just goes back to Junie even if I choose Codex at the bottom of the AI Assistant panel.

Aside from downgrading, is there a way to get the old behaviour back? Also, can someone tell me how to change which agent handles the prompts when I enter them using the Ctrl + \ shortcut?

Thanks