Why is this Index Scan so slow?

From what I can see, the index is not entirely loaded up in memory, and that's what causing it to be slow? Am I correct?

obs: I changed the "Output" section a little bit for privacy concerns

->  Index Scan using idx_customers_orders on public.customers  (cost=0.57..20.28 rows=1 width=13) (actual time=1.757..1.774 rows=1 loops=46257)
                           Output: customers.id, customers.created_at, customers.created_by ,customers.registration ,customers.name ,customers.description ,customers.address ,customers.location ,customers.code
                           Index Cond: (customers.i_order = orders.i_order)
                           Filter: ((NOT customers.deleted) AND (customers.i_databases = (current_setting('db.tenant'::text))::integer))
                           Rows Removed by Filter: 0
                           Buffers: shared hit=164654 read=68033
                           I/O Timings: read=80791.000

The index scan is fast (1.774 milliseconds on average). The execution is so slow because the index scan is executed 46257 times. Slightly over 1 millisecond for reading an 8kB-block from disk isn't so bad.

Most likely, the index scan is on the inner side of a nested loop join. Perhaps a different join strategy would be better; since you don't show the execution plan, we cannot tell.