Slow progress of Time machine backup I am backing up my hdd with the Time machine. Even small incremental backups take several hours. For an hour or so I am in the state "50.3 MB of 496.3MB", while in Activity Monitor I notice that mds_stores has accumulated already over 116 GB data read, and it increases by about 50 MB/s.   My Hdd is this new pcie-sdd type, so it should be efficient with this sort of backup.
Moreover in Console it is written that about 2300 files constitute this 0.5 GB of backup. 

I am backing up to external usb Toshiba drive.

Am I missing something obvious? This is not normal, is it? Can I see which of these 2300 files the Time machine is currently backing up? The "preparing" phase was not so long - took about 30 minutes.
I was having the same problem pretty regularly. I tried the usual tricks (stop backup, eject, mount, delete .inProgress files, stopping spotlight, changing the throttle priority (sudo sysctl debug.lowpri_throttle_enabled=0). Sometimes, this works. Other times it doesn't and it keeps crawling along at a few MB per hour.

One additional trick, that works more frequently, is to force quit the backupd process directly from the Disk tab of Activity Monitor. Then tell it to start backup. This usually works for me. Heads up: this may be a terrible idea in other ways (e.g. maybe it could damage existing backups). In my experience, it hasn't so far.