I don’t understand the problem with NextGen. For a “gallery page” it creates thumbernails which are small (in fact as you can set the size up as you want it). The same goes for opening images themselves – you can set up the resize option for the full size size of images.
Example – it means that if you upload a 5000x5000px image, you can tell NextGen to automatically create thumbernails that are 150x150px, and to shrink images to 1000x1000px.
I mean, if you have really huge photos and you do not resize them, it will take some time for the browser to load them up, there is no going around that..
Or maybe what you want is to have auto thumbnail and image resizing (150x150px thumbs, and 1000x1000px) for viewing in the browser, but to also keep the option of downloading the original full size image?
Perhaps plogger or zenphotopress might better suit your needs.