Time machine backup hangs I do backups of Mac OS High Sierra 10.13.6 with Time Machine to USB external HDD's and at some point I couldn't wait for any backup to complete.
For example it can be like this for maybe over 6 hours, until I just press "Skip This Backup" 


If I check sudo fs_usage backupd, it's just lots of these lines

09:55:49.515864    RdMeta[ST3]     D=0x3e5fe8a8  B=0x2000   /dev/disk3s1                                                                                              0.016663 W backupd.824910
09:55:49.533957    RdMeta[ST3]     D=0x3cafd768  B=0x2000   /dev/disk3s1                                                                                              0.018043 W backupd.824910
09:55:49.565517    RdMeta[ST3]     D=0x3af3e360  B=0x2000   /dev/disk3s1                                                                                              0.031517 W backupd.824910
09:55:49.575010    RdMeta[ST3]     D=0x2c4e29b8  B=0x2000   /dev/disk3s1                                                                                              0.009424 W backupd.824910
09:55:49.591100    RdMeta[ST3]     D=0x399f9a78  B=0x2000   /dev/disk3s1                                                                                              0.016015 W backupd.824910
09:55:49.610730    RdMeta[ST3]     D=0x399a8458  B=0x2000   /dev/disk3s1                                                                                              0.019583 W backupd.824910
09:55:49.634997    RdMeta[ST3]     D=0x366a0e50  B=0x2000   /dev/disk3s1                                                                                              0.024226 W backupd.824910
09:55:49.675542    RdMeta[ST3]     D=0x2fd33260  B=0x2000   /dev/disk3s1                                                                                              0.040490 W backupd.824910
09:55:49.696395    RdMeta[ST3]     D=0x2d39c3d0  B=0x2000   /dev/disk3s1                                                                                              0.020778 W backupd.824910
09:55:49.704406    RdMeta[ST3]     D=0x2c560448  B=0x2000   /dev/disk3s1                                                                                              0.007943 W backupd.824910
09:55:49.722183    RdMeta[ST3]     D=0x2a9fa720  B=0x2000   /dev/disk3s1                                                                                              0.017714 W backupd.824910
09:55:49.752376    RdMeta[ST3]     D=0x00847a20  B=0x2000   /dev/disk3s1                                                                                              0.030050 W backupd.824910
09:55:49.794376    RdMeta[ST3]     D=0x417a1b50  B=0x2000   /dev/disk3s1                                                                                              0.041950 W backupd.824910


occasionally interrupted by 

09:55:42.201947  getattrlistbulk                                                                                                                                      7.885756   backupd.824910


UPD:
tmutil listbackups doesn't show any .inProgress entries, so deleting them couldn't work for me

My current backup session is frozen on 919.2Mb. tmutil status output is:

Backup session status:
{
    BackupPhase = Copying;
    ClientID = "com.apple.backupd";
    DateOfStateChange = "2019-04-13 21:47:32 +0000";
    DestinationID = "80DD4AE0-6BD5-4E91-9A67-2C3346B95A68";
    DestinationMountPoint = "/Volumes/yellow backups";
    Percent = "0.01760709905511998";
    Progress =     {
        TimeRemaining = 143771;
        "_raw_totalBytes" = 46987386344;
        bytes = 919235073;
        files = 548346;
        totalBytes = 51686124978;
        totalFiles = 1048225;
    };
    Running = 1;
    Stopping = 0;
    "_raw_Percent" = "0.01956344339457776";
}


The output doesn't seem to update over time

UPD2: after approx 18 hours of hangup, backup has completed and now it's on Cleaning up... phase.
I'm still wondering what's causing this behavior and if I can somehow avoid it.
In my case, I confirmed the drive was working fine and could easily do almost 100MB/s, yet backups were happening in KB /s.  I had backups going back 4 years and it seems it is the layering of backups that slowed things down.

A few possible solutions:


Many users report that booting into "Safe mode" will reset some data and then restarting back into "Normal mode" may solve such issues.
Try to repair the external disk using "Disk Utility" (may take a very long time).
Can try updating the drive firmware.
If you have enough space on the external drive, partition it into 2 separate disks. (May also take a day or so)


Then setup Time Machine to use the new partition. This will require a complete "Initial" backup, but in my case, 300GB initial backup was still faster than the 4GB incrimental backup on the old drive.

As a final resort, wipe the drive and backup from scratch or buy a new drive to use in parallel.


I'm still experimenting, but there may be a bug causing Time Machine to slow down significantly as new backups are added. I'm using an HFS+j external USB3 drive from WD. Hope that helps.