Thanks for your post.
It would be good to take a look at the SQL Backup logs to see what sort of throughput you were getting on the normal database when performing a backup versus what sort of throughput you are getting now.
If there's a significant drop from when you had the DB running normally to running under Storage Compress, we would probably have to do some testing here and then learn a bit more about your environment, i.e. was the DB also being updated during the backup being taken on the DB running under Storage Compress, and it would also be good to get an idea of I/O reads and writes from both the HyperBacSrv.exe and SQBCoreService.exe through Task Manager.
Of course, you could also try backing up using *.HBC file format to see if you experience a better or similar performance too, but it's not a known issue at the moment with that as a workaround.
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