Been thinking of uploading stuff again but uploading one by one can be a real slog.
Is a mass uploader is the works or is there a separate plugin for that?

For now, there is no such feature.
Partly because we do not want bad actors to spam the site, and because mass uploading often leads to bad tagging.

If you set up a rhythm and process, then semi mass uploading is less of a slog.
I usually do this by opening multiple upload tabs (scatbooru.co.uk/?page=post&s=add), adding each file I want to upload by dragging from folder, and then uploading them, before fixing the tags. If the images are already up on another booru like e6, sankaku, ect, then I will usually copy the bulk of the tags and adjust them as needed before finalizing the upload.

Sounds more clunky than it is in practice :)

It should be mentioned that mass uploading is a planned feature, but considering how much of a pain in the ass this site is to develop for, and the fact that we aren't exactly being paid to do it, means that it's probably not gonna be a feature for a while.
Also, as Kat says, we have to do it in a way that encourages good tagging, which is not particularly easy.
Doesn't mean we're never going to do it, but yknow. We do have lives.