I would only add that I just upgraded from Endnote x7 to x8 today, and suddenly I am having the same lag issues as others describe. No issues with x7, but I was forced to upgrade as the last Word upgrade is not compatible with x7 even though I just bought x7 less than 6 months ago :(.
I am now preparing to revert back to the previous version of Word so I can continue to use Endnote x7 as I see that others have this problem and no fix is in sight.