\copy (select * from oracle_remote_table) to /tmp/tmp with binary - 1 hour and 35 minutes
\copy local_postresql_table from /tmp/tmp with binary - Didnt run because the remote oracle database is currently under maintenance work.
The "\copy...from" doesn't depend on oracle, it would be only depend on local file system (/tmp/tmp), provided that the "\copy...to" finished. Anyway, given the length of time it took, I think you can conclude the bottleneck is in oracle_fdw itself, or in Oracle, or the network.
dumping from Oracle is not fast - I seen it when oracle_fdw or ora2pg cases.