Why does Outlook 2010 give the message "Creating a new item from the selected items could take some time...are you sure you create a new item...?

I had this same problem yesterday and it has something to do with a calendar item in the folder you are trying to move the items from. I selected everything except for this calendar item and it worked fine without the error.

I deleted the calendar item and moved the remaining items to my archive folder.

Microsoft has provided official guidance relating to this issue: Moving multiple items from Deleted Items creates confusing warning about creating new items

It does indeed have to do with the fact that you are trying to move non-email message types (such as meeting requests etc.). Which, coincidentally happens when you are selecting more than 800 items at a time.

This is such a strange behaviour that it could be said this actually is a bug in Outlook, persisting to version 2016 nonetheless. Perhaps it is deemed a low prioity by Microsoft, reason why it takes such a long time to fix.

Compounding the problem is that if you press "yes" in the diologue box that you show, there is no going back if you chose the "move" command (or just dragged e-mails from the folder to another folder). If there are a lot, Outlook freezes for a while and an e-mail gets created with the items as attachment. The items are lost of you don't save that e-mail. You can save all the attachments into a file system .msg file, but dragging and dropping them from a file system folder into Outlook opens up the same problem and the dialogue box appears again.

What's even worse, I think that some of the items that are movable are not created into an attachment but are actually moved (but then again not all of those), leading to a very inconsistent state of your items.

What I chose as a solution (after trying many other things, including some VBA script) is to keep the messages stored on the file system as .msg files, along with a readme file of where they are supposed to go and hope for a future bug fix by Microsoft, hopefull within a few years. Not much of a solution, but the only one that I am able to do right now. Every new major version I will try to import them again. I might update this answer if successful.