Make sure there are indexes on the columns in the ON or USING clauses. Consider the join order when adding indexes. If you’re joining tables A and B on column c and the query optimizer decides to join the tables in the order B, A, you don’t need to index the column on table B. Unused indexes are extra overhead. In general, you need to add indexes only on the second table in the join order, unless they’re needed for some other reason.

Try to ensure that any GROUP BY or ORDER BY expression refers only to columns from a single table, so MySQL can try to use an index for that operation.