I'm the author of both those products... So you can blame me for any frustrations you have...
Which version of wildcard.foldercontents? In 2.x, it should only upload one at a time. I think in 1.x, it would possibly do 2 at once.
By default, collective.documentviewer does the conversions synchronously. Conversions, depending on the size of the PDF, can take quite a long time.
Now if you're only running one zeo client, which defaults to 2 threads, and upload 2 files simultaneously potentially, then attempt to browse the site after, yes, it will be very slow.
Solutions
turn off automatic conversions of PDFs. There should be an option in Site Setup. You will have to later manually convert the document to documentviewer though.
or integrate plone.app.async which will do the PDF conversions on a worker zeo client so you won't get bogged down after uploading a bunch