I just replaced a mid-2015 iMac with a new 2019 iMac. The new model has a faster processor, a faster Fusion drive and a faster video card but has only 8GB of RAM compared to the older model which had 16. I have my Photos library on an external USB 3.0 Western Digital drive (not an SSD drive) and it is pretty large (85,000 items, total size about 690GB of 3TB total for the drive). I have noticed since transferring everything to the new iMac that while just about every program (including Windows 10 running under Parallels, Microsoft Office and Pixelmator (Original and Pro) run much faster than before EXCEPT Photos. There a lots of pinwheels - like when having a picture open to larger view (double clicking), opening the Edit screen or trying to us the Edit In command to Pixelmator Pro. Much more pinwheels and hesitations than when the same drive was plugged into the older iMac. Could lowering the amount of RAM from 16-8 be the cause? I have tried running Photos with Activity Monitor running and the little graph of Memory Pressure never changes from Green to either Yellow or Red. I do realize that this is a large library to keep on an external HD - especially a non-SSD one - but Photos (as opposed to every other app I have used) seemed to work much better (again same external hd) on the older machine. Any Suggestions? Thanks.