Simple DELETE on modest-size table runs 100% CPU forever

I'm completely baffled by this problem: I'm doing a delete that joins three
modest-sized tables, and it gets completely stuck: 100% CPU use forever.
Here's the query:

explain analyze
select count(1) from registry.categories
where category_id = 15 and id in
(select c.id from registry.categories c
left join registry.category_staging_15 st on (c.id = st.id) where
c.category_id = 15 and st.id is null);

If I leave out the "analyze", here's what I get (note that the
categories_staging_N table's name changes every time; it's
created on demand as "create table categories_staging_n(id integer)").

Aggregate (cost=193.54..193.55 rows=1 width=8)
-> Nested Loop Semi Join (cost=0.84..193.54 rows=1 width=0)
Join Filter: (categories.id = c.id)
-> Index Scan using i_categories_category_id on categories
(cost=0.42..2.44 rows=1 width=4)
Index Cond: (category_id = 23)
-> Nested Loop Anti Join (cost=0.42..191.09 rows=1 width=4)
Join Filter: (c.id = st.id)
-> Index Scan using i_categories_category_id on categories c
(cost=0.42..2.44 rows=1 width=4)
Index Cond: (category_id = 23)
-> Seq Scan on category_staging_23 st (cost=0.00..99.40
rows=7140 width=4)

The tables are small. From a debugging printout:

7997 items in table registry.category_staging_15
228292 items in table registry.categories
309398 items in table registry.smiles
7997 items in joined registry.category_staging_15 / registry.categories

What on Earth could be causing this simple query to be running 100% CPU for
hours?

Postgres: 10.10
Ubuntu 16.04
This is a VirtualBox virtual machine running on a Mac host.

Everything else seems to work as expected; just this one query does this.

Thanks,
Craig