Update table performance problem

I am trying to update a field in one table with a
field from another table like:

update co set
firest_id=fco.firest_id,fire_dist=fco.fire_dist from
fco where co.xno=fco.xno

Table co has 384964 records
Table fco has 383654 records

The xno fields in both tables are indexed but they
don't seem to be used. I would expect the update to
be faster than 6.3 minutes or is that expectation
wrong? Here is the results of Explain Analyze:

"Hash Join (cost=15590.22..172167.03 rows=383654
width=215) (actual time=1473.297..43032.178
rows=383654 loops=1)"
" Hash Cond: (co.xno = fco.xno)"
" -> Seq Scan on co (cost=0.00..123712.64
rows=384964 width=195) (actual time=440.196..37366.682
rows=384964 loops=1)"
" -> Hash (cost=7422.54..7422.54 rows=383654
width=34) (actual time=995.651..995.651 rows=383654
loops=1)"
" -> Seq Scan on fco (cost=0.00..7422.54
rows=383654 width=34) (actual time=4.641..509.947
rows=383654 loops=1)"
"Total runtime: 378258.707 ms"

Thanks,

Fred

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