I have currently less than 1,000 references (with PDFs) brought over from another citation manager to give EndNote X9 another chance. The display of PDFs is not the only culprit for sluggish behavior. I have also the right pane set to “Right - Preview”. But switching from one reference to the next shows the beachball each time for about 3-4 seconds before the contents of the right pane is finally shown – on a current MacBook Pro with 16 GB of RAM running macOS 10.14.5. I do not know which database engine EndNote X9 is using, but this is something, which has to be improved with the highest priority.