I see this all the time as well.
Today, I uploaded a zip file with 371 images using the original PBase upload page. An hour later, it was still spinning.
Looking at the content of the gallery in another tab, I see I have a wildly messed up sequence, and in many cases, 2-3 duplicates of the same image.
I selected & deleted the content, then started over with the new uploader. In this case, I sub-divided the images into batches of 50 each. While uploading the first batch, the tool showed the upload progress (thank you) and then the unzip process. It apparently unzipped it 3 times, but while most images have 3 copies, some have two, others, one. I stopped at that point on this gallery.
Stats:
MacOS 10.8, Firefox 47.0 (Cannot upgrade to newer version as that's the last available on this MacOS version)
Images processed with Graphic Converter 8.8.3, sized to 1000 px in the long direction.
Sample results:
http://www.pbase.com/brucemac/2017septmunich. This is after I re-sequenced by image name in the new uploader tools.
I've had issues with the uploader several times in the past as well. In those cases, I've spent the time cleaning & rearranging to straighten things out. I'm not sure what's going on, but it's becoming a pain to upload any more than a few images at a time.
I'll try again from a windows 7 VM with the same zip files, but I'm not holding my breath.
-BruceMac