Since you are willing to break the all one transaction rule, and if the restores were created via pg_dump, you could use the --section argument to split things up, run the "pre-data" sections serially, and the rest ("data" and "post-data") concurrently.
Cheers,
Greg
--
Crunchy Data - https://www.crunchydata.com
Enterprise Postgres Software Products & Tech Support
