I guess nobody can say if the migration will speed up your sync, because there are just too much possibilities why it is slow.
As far as I know the owncloud desktop client is still compatible with nextcloud, kick me if I am wrong.
Just make a full backup of your owncloud instance including databases etc etc and try it out. I guess you need to adjust the owncloud version number in your config.php back to 10.0.0 to be able to upgrade. Some admins faced problems while trying to upgrade from even oc 10.0.1 to nc 12, because the internal version string of the last oc is higher than last nc or something: Migrate owncloud 10 to nextcloud 12
The migration page also says that upgrading from 10.0.1 is not yet working, which should include 10.0.2: https://nextcloud.com/migration/