This has been a longstanding problem-you ain’t alone! Yet nothing has been done since it started with X8 and clearly persists in X9. I just gave up and switched to Bookends. Fast, stable, with even more features than Endnote and way cheaper. There are many alternatives out there and I’m just bummed I had persisted with the slowness nonsense in X8 for two years. No reason to do that, especially when the developers have no clue how to fix it, just blame Apple and still haven’t made this a 64-bit app. So you might want to consider switching to Bookends, Zotaro, Mendeley, etc. Tons of options out there.