I kind of meet the same issue. Selec ting several images all I can get in terms of “by lot” is the border buffer: any adjustment like Exposure, Brightness, Contrast, etc works only on the first image of the lot.
As a general point I also find V3 slightly slower than V2
What you are describing is the expected behavior. All of the pre-conversion settings will be applied to all the images you have selected. But any editing of the settings on converted negatives will only happen on the primary selected negative. If you want to bring over your new exposure, brightness, contrast, etc over to the rest of the images, that is what the “sync” function is for.
If you are finding it slow, make sure you are in the “Develop” module. It will run in the “Library” module, but it will be considerably slower (because it is using a different part of the Lightroom API.
In general, the startup time for Negative Lab Pro will be just a bit longer (because it is now loading presets and roll history), but once a session has started, it should be as fast as v2.