Time Machine backups slower in Sonoma than Monterey

The behavior you're seeing is not something we'd generally expect after upgrading from Monterey to Sonoma, but there are several factors that could contribute to the increased backup times.

• One of the most significant changes introduced in recent versions of macOS is the evolution of the APFS file system and Time Machine (TM) itself. Sonoma performs more extensive tracking of file system metadata, snapshots, and extended attributes than earlier versions. In some environments, this can result in Time Machine detecting substantially more changed items than were reported under Monterey, even when relatively little user data has actually changed.
• Another consideration is the age and size of the backup history. With approximately 2.7 TB of backup data spanning four years, TM has a very large backup database to maintain.
• Your storage hardware may also be contributing to the issue. Although the Seagate drives are perfectly suitable desktop drives, they are 5400 RPM disks, which offer relatively modest random I/O performance. TM performs a tremendous amount of small file and metadata operations rather than large sequential writes, so rotational latency becomes a significant bottleneck. Also, how the drive enclosure is connected to your Mac can be a limiting factor.

Regarding Disk Utility First Aid, a runtime of approximately 12 hours on a mirrored 2.7 TB volume made up of 5400 RPM drives is not necessarily abnormal. Since the second run completed without reporting additional issues, it suggests the file system itself is likely healthy.

As for whether upgrading to Sequoia would improve the situation, AFAIK, there isn't any evidence that Sequoia significantly changes TM performance in either direction. Here, at the ASC, some users report marginal improvements, while others see similar behavior to Sonoma. Based on your description, I would not expect Sequoia alone to reduce backup times from 8–12 hours back to the few hours you experienced with Monterey.

With all that said, here are some troubleshooting steps that may help:

• Verify the RAID volume's SMART health (if your enclosure exposes SMART data).
• Allow Spotlight indexing to fully complete after the OS upgrade.
• Check whether antivirus, cloud sync utilities, or other background applications are modifying large numbers of files. If your Mac is using a third-party AV app, disable, or better still, completely remove it.
• Review the TM logs to determine whether it's spending most of its time scanning, preparing, or copying files.
• If you're considering a hardware upgrade, replacing the 5400 RPM drives with SSDs (or at least 7200 RPM NAS-class drives) would likely provide a much more noticeable improvement than upgrading from Sonoma to Sequoia.