Since I transfered from Version 6.x to 7.x of EndNote I encountered a substantial slow down of my operations in the database with nearly 16,000 entries. I tried many solution, thereunder those in this forum and on other web pages. I monitored the internet-traffic as I thought EndNote might connect endlessly to the internet. I completely rebuild the database. Nothing worked so far – always very slow operations when I worked within the database (no Word connection).
Today I finally found the solution to the problem. 
What slows EndNote down are the Smart Groups. I deleted nearly all of my 12 Smart Groups (3 are left) and now EndNote runs as it ran before the update from 6.x to 7.x.
Yet, this is now a handicap for my work, because the Smart Groups substantially helped me in organizing the work I do.
Therefore, I think that the programmers have to have a close look into this issue.
I hope that helps others to solve the issue as well.