Here are two main causes of a slow backup process:
1. The disk I/O of the web hosting is slow.
We have noticed that the disk IO of many web hosting is only 1MB/s, then theoretically backing up a 100MB website would require at least 300s. You can check the disk I/O of your web hosting by asking your web hosting provider or through cPanel.
2. Some server-resources-friendly WPvivid settings are enabled.
These options are designed to boost backup/migration success rate on web hosting with inadequate resources. Therefore, backups would be running slower when they are enabled.
With that being said, you can still try to adjust the WPvivid Settings as suggested in this doc to speed up a backup process.