With a 1 Gbps network you can achieve a bit over 100 MB/sec at best. So 50 MB/sec isn’t too bad at all considering that Duplicati still does a lot of processing. There is no way to disable the chunking, deduplication, and repackaging of chunks into volumes. Encryption and compression add even more overhead so disabling those might help. Also keep in mind some Duplicati operations are single threaded so that may also be a factor limiting your performance.
Once the initial backup is complete your later backups will be much faster.