[SQL] Frequent update of users in production, slow query

The following query runs very often on a produciton system. It might be jsut legitimate logins that trigger an update, but as the query is also very slow (avg. 3 seconds per run) we should find out where it comes from, why it is run and why it is so slow:

UPDATE "auth_user"
SET "password" = $1,
    "last_login" = $2::timestamptz,
    "is_superuser" = $3,
    "username" = $4,
    "first_name" = $5,
    "last_name" = $6,
    "email" = $7,
    "is_staff" = $8,
    "is_active" = $9,
    "date_joined" = $10::timestamptz
WHERE "auth_user"."id" = $11