SELECT vs UPDATE performance with index

Question

If I SELECT IDs then UPDATE using those IDs, then the UPDATE query is faster than if I would UPDATE using the conditions

Answer 1

The two queries are not identical. You only know that the IDs are unique in the table.

UPDATE ... LIMIT 10 will update at most 10 records.

UPDATE ... WHERE id IN (SELECT ... LIMIT 10) may update more than 10 records if there are duplicate ids.