Over time, the period required for Time Machine to make its hourly backups gets steadily longer. What used to take less than a minute grows into a few minutes, and the hefty backup required after a macOS update can take more than ten minutes.
This occurs even if you keep ample free space on your backup volume. One reason is that your backup volume is getting clogged up with millions of ‘files’, the vast majority of which are hard links, whose purpose is to make each backup look like it’s a complete copy of the original, at that moment.
Once you have sufficient determination to do something about it, here are some ideas which you might like to explore.
Time Machine tries to eke out the available space on its backup volume, but the harder that it works to do that, the longer backups become. This is because some time is spent deciding which old backups can be deleted, and removing them. When space gets tight, and Time Machine gets more aggressive at deleting old backups, they inevitably take longer to complete.
2. Speed of access to the backup volume
Like all document storage, the faster the hardware, the quicker access will be. If you can afford it, get a large hardware RAID array of SSDs and connect it via your fastest supported Thunderbolt interface. However, the HFS+ file system is not known for its blistering performance on SSDs, and not all Thunderbolt storage is any quicker than USB3, so it is easy to spend a great deal in the pursuit of very little.
A good hardware RAID array is not only quick, but if you select the right RAID level it should be robust in the face of failure and errors too.
3. Don’t run your backups longer than a year at most
Until the release of High Sierra, this Mac backed up to a 6 TB hardware RAID which contained backups for the last five years, but was little more than half full. Each backup required a lot of disk activity, and took minutes rather than seconds. Once I re-formatted the RAID and started backing up to a fresh volume, very few backups (apart from the first, of course) take longer than a minute.
Work out a plan for making an archival copy of each year’s backups, and every twelve months start a fresh backup on a newly-formatted volume. It makes a huge difference: the drives don’t thrash about when backing up, and restoring items from backup is also far quicker.
4. Add large files to the exclude list
Time Machine doesn’t back up small changes within very large files, but all changed files. If you have a 5 GB movie file which undergoes just a tiny amount of change, each time it changes the whole file will be backed up. Try to keep it in a separate folder, and add that folder to Time Machine’s list of excluded items. You will then need to make a manual copy of those large files when you want to back them up.
Bundles are a hierarchy of folders and files, cunningly disguised to look like a single file, like your Photos Library. Because Time Machine sees the constituent files and folders making that bundle up, it only backs up those files and folders which have changed. This is much more efficient than a single huge image database file, but it also increases the number of files and hard links on the backup volume, and helps clog it up over time. What you save in space you eventually lose in performance degradation.
5. Maybe APFS?
Currently, your Time Machine backup volume (when local) must be in HFS+ format, and cannot use High Sierra’s APFS. As the latter is intended to be a modern, high performance file system, it may eventually support more extensive backups than HFS+ does at present. However, it is also primarily intended for SSD storage, and still has a way to go before it performs at all well on hard drives. It also doesn’t support hard links, which Time Machine currently relies on.
Perhaps at some time in the future, a new version of Time Machine which works with APFS volumes will do better.