I'm guessing you have an iPhone or iPad (likely iPad) with a mobile version of this app, right? You are asking how to bulk transfer the items? If that's correct, here's a couple options:
1) Put all the files in a single archive file and transfer that. ZIP, RAR, etc. Here is a link to one possible app that will let you do that: http://www.izip.com/ (I'm not endorsing it, just giving you an option). Now if the bottle neck is your internet speed, this solution may not help much. If the issue is the number of files, it may help.
2) Another alternate is to tether your device to your iMac with a cable.
If you can provide me with a little more information, I'll be happy to clarify my answers and/or provide you with more troubleshooting.
If this was enough to solve your issue, awesome! In that case, I would greatly appreciate it if you would rate me! If you need further help, just reply here and we'll get this issue solved!