I did add this as a bug, but it dos not look like, it's more important to maintain a face of we support multisites, then correcting a huge problem, proberly effecting quite alot.
Multisite support is not compleate, since domain mapping is not supportet, and with out domain mapping, most multisites setups are useless, unless you got a lot of blogs called blah.siteurl.com.
Personal, i use Multisite to host every site i got, or make for costumers, since it's easy, direct, and i can keep it updatet from one place.
But Domain Mapping, is killing NextGen Gallery, the speed slows down to painfully slow, 30-60 sec load time for each page.
Hope this trows a light for all thos with speed issues, if you are on a multisite, and got/use domain mapping NextGen Gallery is a big NO NO.