mysql innodb dump restore slows down after upgrade mysql 5.7 to 8.0
innodb_log_buffer_size should be no more than 1% of RAM.

key_buffer_size should be only 30M, assuming you do not have any active MyISAM tables.

Those changes will let you increase innodb_buffer_pool_size to 7G, which might help.

Do you have any FULLTEXT or SPATIAL indexes?  FOREIGN KEYs?  Stored programs (probably not, since you did not ask for them to be dumped)?

If those don't help, file a bug with https://bugs.mysql.com