In CRM_Standaloneusers_BAO_Session::read, which is called on every request, we see this query:

$stmt = $db->prepare("SELECT * FROM $table_name WHERE session_id = ? FOR UPDATE");

RPOW classifies this as an rw query and replays everything executed to that point on the r/w db and sets a write cookie.

I think this means we're not actually getting any web requests fully executing on the r/o database, which might explain some of the performance complaints about page loads we're hearing lately.