For the past few months, I’ve had a heck of a time with Lightroom Classic running slowly on my iMac. While my iMac is from late 2015, it’s fully loaded. Tons of memory, a good graphics card and a modern-day processor. But it’s had its share of problems too, between random restarts and a Fusion drive that slows things down.

The Fusion Drive problem

When Apple first introduced Fusion Drives, it was a great solution to get a fast solid state drive (SSD) coupled with a traditional hard drive, for a combination of speed and storage. The problem is that the solid state drive portion of the overall Fusion Drive is pretty small — it basically holds macOS and a few other apps or documents that you launch often. There’s no way to tell apps, documents, Lightroom catalogs, etc. to launch via the SSD.

And in my case, if you have a rather large Lightroom catalog, it exceeds the size of the SSD as it is. This means some major slowdowns when doing simple Lightroom Classic tasks like importing, exporting, browsing and editing.

The SSD solution

After talking to Photofocus Managing Editor Kevin Ames and Adobe’s Tom Hogarty, I decided to go out and purchase a 500 GB external SSD. I would store my Lightroom catalog on the external SSD instead of my iMac’s Fusion Drive.

The nice thing with this is that, if I really wanted to, I could take this SSD on the road and connect it to my laptop (as long as I have a backup).

Once I got everything moved over, I did notice a good speed boost. But I wanted to see if I could push it even more.

Increase and move your cache

Whenever you edit RAW photographs inside Lightroom Classic, high-quality previews are created. This process is faster if the original image data is in your Camera Raw cache. The default cache size is 1 GB, but you can increase this, which in turn speeds up the building of these previews. Go to Lightroom, Preferences (or Edit, Preferences on Windows) and click the “Performance” tab. In the Camera Raw Cache Settings area, increase this to at least 10 GB (for reference, I have mine set to 30 GB).

Furthermore, make sure you move your Camera Raw Cache to the SSD drive you just set up — this should speed some things up too. You can do this in the same Performance tab, where it says “Location.” Point this to a separate folder you create on the SSD drive.

Build your previews at a lower resolution

This might seem counter-intuitive, but it actually works. The issue with the iMac is that it has a 5K display. And if you have your iMac set to the “More Space” display setting, Lightroom is building giant previews which take up a ton of space (and therefore slow down the program).

I suggest turning your iMac to the default display resolution — this actually gives you the sharpest view on everything. If you want to take it a step further, go to your Lightroom Catalog Settings (Lightroom, Catalog Settings… on Mac or Edit, Catalog Settings… on Windows), click on the “File Handling” tab and change the Standard Preview Size to be one of the default options.

The downside here is this might affect what you see when editing. If you tend to zoom in a lot on your images, this won’t be ideal for you — in this case, choose the “Auto” size, as this will create the previews based on your screen resolution. But if you don’t find yourself zooming in much in Lightroom, then this method totally works.

Don’t be fooled by that 15 GB total size — this was captured about 10% into my preview re-build! This can be the biggest file in your Lightroom folder on your hard drive or SSD.

From there, exit out of Lightroom. Find your catalog and delete the previews file (Lightroom Catalog Previews.lrdata). Then, open Lightroom back up and go to Library, Previews, Build Standard-Sized Previews. You can either select a number of photos in your catalog or have it build previews for all the photos.

This takes quite a bit of time, so it might be something you want to run overnight (or over the course of several nights). Even without previews built, you can use Lightroom, as it’ll build previews as it generates the thumbnails and when you open photos in the Develop module.

After rebuilding all my previews, I must say that my Lightroom feels like a brand new catalog, despite having over 200,000 photos in it! While the Fusion Drive issue seems like a simple problem and one that I hope Adobe can fix soon, for now, these few steps have made my life in Lightroom Classic a little more speedy.