There have been some improvements in the past and each version there should be some improvements. This time, they especially pointed out the LDAP performance (Nextcloud 13 Brings Improved UI, Video and Text Chat, End-to-end Encryption, Improved performance and more).
I don’t think that NC 13 will speed up the thumbnail-generation a lot. You can reduce their size, so it’s perhaps a bit faster:
https://docs.nextcloud.com/server/13/admin_manual/configuration_files/previews_configuration.html?highlight=thumbnail
If you uploaded files during the day, you can use this app to generate previews during the night:
https://apps.nextcloud.com/apps/previewgenerator