I am also having this trouble. Only working with records in a library without inserting citations into Word or any other applications, I find that I can open between 5 and 10 records before the application begins to slow. Before long, it really begins to lag. Has a solution for this been found yet?
I am using Endnote X8.1 on PowerMac OS 10.11.6
Library Name: Flag Debates.enl
Location: /Users/jmontezd/Documents/School/Projects/Active/Flag anthem debate/Flag Debates.enl
Last Saved: Thursday, December 14, 2017 at 3:30:10 PM Mountain Standard Time
Records: 356
Custom Groups: 0
Reference Types Used: 2
Most Used ref. types: Web Page, Electronic Article
File Attachments: 0
Figures: 0
Authors Term List: 2
Journals Term List: 0
Keywords Term List: 5