The bottleneck source means the disk storage reader component spent most of the time reading data from the storage.
You can review a detailed description of the bottlenecks stats in the sticky thread under the bottleneck analysis
Do you have another host and did you try to backup another VM from different storage? We should understand if the storage has an issue or this is the host problem or something else.
I'd check the firmware and make sure you're using latest storage drivers.
Also, it's recommended to use dedicated backup proxy servers.
Please read the discussion about the similar issues on the flash arrays
, as it contains a few suggestions on how to improve it.
Do not hesitate to contact the support team in order to let them take a closer look at your environment, also they will run a default test to check reading speed of the source. Thanks!