When profiling a backup with many small volumes, a large portion of the time spent went to generating the AES header IV. This blog post describes the problem, the solution, and the performance improvements. The solution has been merged in the sharpaescrypt project in pull request #1 and published in the nuget package SharpAESCrypt 2.0.3, which is now used in Duplicati in the merged pull request #5597.

The AES header IV generation was slow as it queried the operating system for a MAC address on every call (and it had an error, resulting in a default value being used). Fixing the error and caching the MAC address improved the performance by up to 1.85 times. This led to encryption being essentially free for this particular backup.