How much faster will 7200 RPM drives be than 5400 RPM drives for Lightroom use with over 500k photos

Those OWC ThunderBays are great for mass storage. But 7200RPM vs 5400RPM may not make much difference. The ThunderBay advertises up to 2586MB/sec throughput over Thunderbolt, but that is only when using SSDs, and maybe only if the SSDs are set up as a RAID.

If you will use hard drives, realistically the maximum throughput will be around 150 to 200MB/sec depending on the data, maybe slightly more as a RAID. Affordable hard drives just don’t go faster than that. (The slowest SSDs are around 350-400MB/sec, but more commonly 500MB/sec and up.)

The way previews are displayed is:

1. Read the preview if it exists. (The previews are in the same folder as the catalog.) If the preview hasn’t been built yet, read the original wherever it is stored, use the CPU to build and compress the preview, and write the preview to storage.

2. Use the CPU to decompress and display the preview.

The reason I put it that way is to show that drive throughput is not the only factor that determines how fast previews and grid thumbnails display. A faster CPU can create, compress, decompress, and display previews faster. And importantly for the grid, a CPU with more cores can do that with more images simultaneously, filling in more grid thumbnails at a time.

After a preview is built for an image, the drive that matters for performance outside of Develop is where its preview is stored (with the catalog), not the drive where the original is stored.

So although faster drives for originals might help, how much it actually helps depends on those other factors. This is why 5400RPM drives are so commonly used in servers. (Also, RPM is not an absolute measure of throughput. Because technology has improved, some of today’s 5400RPM drives are faster than some older 7200RPM drives.)

And, another factor that was mentioned is that maybe there is a Lightroom Classic problem that needs to be fixed regarding grid scrolling. If that’s happening, that would be on the Adobe side to fix, not your hardware.

In short, the 5400RPM drives are probably fine. You linked to WD Red drives, which are designed for long hours of service in an enclosure that is always on (that’s why they’re labeled NAS).

Will the previews scroll significantly faster with 7200 RPM drives?

Lots of people are reporting previews scroll slowly lately. I'm not sure the drive speed is the culprit. As asked by @GoldingD, which drive is the catalog file stored on?

Are you going to place your catalog, or just your photos, or both catalog and photos on that RAID ext drive?