Archive format has small impact on restore time.
That's because actually, when you restore from plain format, it is equivalent to:
cat backup.file | psql emptydb
and when you restore from "compressed" format, it is equivalent to:
pg_restore backup.file | psql emptydb
, and the cost of decompression with pg_restore is usually negligible, compared to write-intensive COPY/CREATE INDEX commands which happen during restore.
For large dumps, restoring from compressed dumps may be a bit faster (less disk reads).
It might be even faster with
--jobs) option. It can be used only with "compressed" format, and it allows paralell restore with N concurrent jobs (sessions).
For more details about
-j option, see http://www.postgresql.org/docs/8.4/static/app-pgrestore.html. Example:
pg_restore -j 4 -d emptydb backup.file