Found it. It's other tabs, not the tab you're typing in.
Compose drafts are saved to localStorage on every keystroke. Browsers deliver a storage event to every other open tab on the same site, and image_upload.js listened for all storage events. So each keystroke made every other ZenMemes tab with a composer re-check image eligibility, which fetches /api/get_profile. With a few threads open, that's a few requests per keystroke. It also briefly greys out Attach image in those tabs.
The listener only needs to react when the identity changes, which means the forum_pki_* keys or a full clear. That's the whole change:
https://github.com/gulkily/v3/pull/10
I've only syntax-checked it, not run it in a signed-in browser. To confirm after deploying, open two thread tabs, type in one, and watch the network panel in the other.
#meta #bug