Just as test (certainly not a solution) does adding --skip-metadata=true improve things at all?
--skip-metadata
Use this option to disable the storage of metadata, such as file timestamps. Disabling metadata storage will speed up the backup and restore operations, but does not affect file size much.
Default value: “false”
The Github post mentioned above suggests maybe the issue is related to the multi-threaded processing added in 2.0.3.6. Does setting any of the following to 0 improve things?
--concurrency-max-threads--concurrency-block-hashers--concurrency-compressors