I notice a few people have been having various problems with NextGen after the upgrade to WordPress 3.4.
Mine was still working but was running as slow as a wet week.
I was using Firefox 13.0.1 as the browser.
I switched over to Chrome 19.0.1084.56 and it truly flies with no delays or problems.
YMMV, but if you are having problems try a different browser to see if that solves the problem.