35 MB/s is not expected. If you run kopia benchmark crypto measures the throughput of hashing and encryption (which should be upwards of 200-300 MB/s single-threaded on modern CPUs).
On top of that there’s also compression (if you use it) and splitting (kopia benchmark splitter) which on my machine is about 150MB/s single threaded.
Kopia can achieve best throughput when the upload is parallelized, which is not implemented yet for individual large files but can and will be in the future.