How long should a merge take? How can I speed up my merge?

Merging libraries (or doing large album/photo copies) can be a very intensive operation, involving a lot of processing and data copying, and thus can take quite a while to complete. There are many factors that can affect how long a merge takes, so it’s very difficult to accurately predict how long a given merge can take, but here are some factors that may affect your merge time and a few ways to try to speed things up.

How long should a merge take?

A “typical” merge involving maybe tens of thousands photos can easily take a few hours to complete. If you have lots of libraries, possibly involving 100,000 photos or more, then a merg can potentially take multiple days in extreme cases. The primary factors that affect how long a merge takes are as follows, with the most relevant first, decreasing in importance as you go down the list.

1. Number and size of photos/videos The primary factor is the size of the libraries being merged, both in terms of the total number of items, and the total size (e.g. in megabytes/gigabytes) of the libraries involved. If you have any easy pruning of photos/videos from any of your libraries that you can do before starting the merge, that can help. Photos/videos that are in the “Recently Deleted” album during a merge are ignored and won’t affect the merge completion time.
2. The number of duplicates If your libraries have a large number of duplicate photos among them, then that will make the merge take longer, because of the additional checking that PowerPhotos has to do to find all the groups of duplicates and ensure only one copy from each group ends up in the merged library.
3. The speed of the drive(s) containing the libraries Merging involves a lot of file reading and copying, so the slower the drives that contain the libraries you’re merging, the longer the merge will take. Generally speaking SSDs are the faster, followed by “spinning platter” hard drives, and finally drives accessed over a network, either using file sharing or a NAS.
4. Using an optimized iCloud library as one of source libraries If you’re merging photos from your iCloud library into another library, and have the “Optimize Mac Storage” option enabled for iCloud Photos, then Photos will need to download all the photos/videos from iCloud during the merge process so that they can be copied over to the merge destination. Using your iCloud library as the destination does not have this same problem.
5. The speed of your Mac’s processor This is much less of a factor than the ones listed above, but the duplicate analysis phase of the merge does involve a lot of computation, so older Macs will tend to be slower analyzing duplicates.

One factor that doesn’t affect merge speed very much is the amount of RAM in your Mac. The merge process is mostly limited by data transfer, and doesn’t use large amounts of RAM, so other than having some other app on your system using lots of RAM while the merge is taking place, system memory shouldn’t be a big factor. But quitting Slack couldn’t hurt.

How can I speed up my merge?

All that being said, here are a few tips you can use to try to speed up your merge.

1. Merge into an existing library When you setup a merge, you have the choice to either merge your libraries into a brand new empty library, or use one of your existing libraries as the destination and copy the photos from all your other libraries into the existing library. Choosing an existing library as the destination means that no photos from that library need to be copied, which can help decrease the total amount of data that needs to be copied during the merge process, and thus speed up the merge. All else being equal, using your largest library as the destination should give you the most speed benefit.
2. Put your libraries on fast drives If possible, putting your libraries on the fastest available drives can speed up the merge process significantly. This can be difficult since often your fastest drive is your internal SSD, which is also the drive most likely to run out of space. At the very least, try to avoid using network drives during the merge process itself, since they are the slowest. For example, if you ultimately want your merged library to be stored on a NAS, you might want to first do the merge on a locally attached hard drive, and then copy the merged library to the NAS once the merge is complete.
3. Put libraries on the same APFS drive Apple’s new(er) APFS drive format has a feature called “file cloning” which PowerPhotos can take advantage of when merging. If both the source and destination library are on the same APFS drive, then photos and videos can often be transferred to the merged library without having to make a separate physical copy of the data on disk, which can greatly speed up the process.
4. Disable duplicate detection if you don’t need it One of the great benefits of merging with PowerPhotos is that it can eliminate duplicate photos during the merge process, ensuring that only one copy of each photo ends up in the merged library. However, if you’re confident that the libraries you’re merging don’t overlap and would not contain any duplicates, then unchecking the “Eliminate duplicates while merging” checkbox can save a lot of time for the merge process.
5. You don’t need to merge all your libraries at the same time If you have a lot of libraries that you want to merge together, the obvious way to proceed is to simply add all the libraries to the merge setup at once and let PowerPhotos crunch its way through everything. This will work, but if the total number of photos is extremely large, it can take a very long time to get through a giant merge like this. One alternative is to do the merge in smaller batches, by only merging one or a few libraries at a time into your merged library. Doing the merge bit by bit will have the same result as doing it all at once, and while the total time from beginning to end may actually be a bit longer, splitting it up into several smaller merges means each individual merge doesn’t take nearly as long, and lets you check in after each step to make sure things are proceeding as you expect.