You have my sympathy.

Look at the plan; it is very well possible lots of data is fetched from
the remote site, to be processed (and discarded) locally.
If so, a possible alternative would be to create a snapshot on the 7.3
side, using the query above, and fetch that using something like
'select * from snapshot@remote_site'.

If (re-)creating the snapshot is too cumbersome, you could partition,
or use a view.

By the way: the first commit is totally redundant; truncate table is
DDL, and commits!

Frank van Bortel