Photos App : Using Photos App on a library that is stored on a USB Flash Drive
You should format the secondary storage as OS X extended and journaled. That ensures the mount of the device is quick and you don't have to wait for a full fsck cycle if it unmount uncleanly. The speed penalty of journaling is almost un-measurable. The speed penalty for not journaling is huge each time you mount the drive.

You want OS X style ACL and permissions, so don't choose FAT or other formats if you care about speed and no bugs/issues.

Here is guidance on the actual move and measurement tools to check drive speeds if you don't simply want to time the Photos app once you've moved the folder.


Moving home folder or large folders to HDD
Can I move applications and other data to a JetDrive 128 GB for MacBook Air