> TM heavily throttles disk I/O used for backing up

That makes sense, and I usually quite like that behavior. I barely ever notice an impact when backups are running.

However, this is happening every time on one machine (Intel iMac), and semi-regularly on another one (M3 MBP), after a fresh restart, giving mds_stores some time to settle down, and the most recent backup just hours ago, with no significant changes on disk since.

In a situation like that, I would expect the "Preparing backup..." stage to just take a second to create an APFS snapshot, and maybe a minute to diff that snapshot against the remote state. But not 10+ minutes.

But thank you for the hint about that sysctl parameter! I will certainly give this a try.