nisus.com

... frequently, but not always, if I move a floating figure around, particularly quickly, the beachball appears and NWP becomes unresponsive. This condition never clears itself. I don't remember this happening in the past, although I haven't used NWP for a few months. The document being edited is only five pages, and there are two floating figures in the "position on page of p" placement setting.

I *am* using 10.10.3 build 14D87h, on a MBP 13" (MacBookPro11,1) just in case that matters

This is not normal. I would suggest that you send the file to Nisus using the "Send Feedback" in the Help menu, and ask them to have a look at it.