Reasons for VSS backups taking so long

The most significant factors are generally the change rate and the client I/O performance. Around 90% of the performance gain from the Avamar client comes from the file cache. If large numbers of files are changing, this will cause file cache misses and the client then has to spend the time and I/O cycles to process and re-chunk the files looking for changes. The bottleneck for backup speed on an individual client is normally the performance of the client filesystem.

The change rate within the files themselves may also have an impact since it takes time to send changed chunks across the network and wait for the answer but this is generally less significant than the change rate or the I/O performance.

Without running specific tests, it's hard to say what the bottleneck in this specific case might be since there could be other factors (strange data patterns on the client, high latency link, etc.) but this should give you an idea about typical behaviour.