Thanks a lot for your persaverance! Although I did not find this thread searching for a solution for slow uploads (my problem was generic slow navigation in Nextcloud), your solution of deactivating http2 (I’m on Apache) did solve my issue.
Had looked into a perhaps faulty redis configuration, I/O performance, CPU load, network config, all to no avail. Any then hit your post, so thanks a lot for posting your finding. My weekend has been made! 