How much time should a backup of a 100Gb database take using streaming server setup?
You are plausibly in the range where network bandwidth is limiting.  I get 9 hours versus your reported 23, but that could be accounted for by overhead, inefficiencies or competing network usage.

You can try turning on network compression, which requires changing the backup method, which in turn will require you to configure "ssh_command".

ssh_command=ssh postgres@192.168.0.15
backup_method = rsync
network_compression=true


Of course this will impose some CPU overhead on the server for it to do the compression.