I may know the reason, and a possible solution.
Ok, let me start by saying that I hope Affinity Designer becomes a real AI contender because Adobe needs to do to Illustrator what it did with InDesign (PageMaker) -- rewrite the entire program for modern operating systems. I use AI nearly every day, and it is such a patchwork of bells and whistles taped to a wheezing ox of bloated ancientware that I can't believe it doesn't crash more.
To your specific problem, two immediate things jump to mind:
(1) AI has friction especially when attempting to IO with the file system. Recently I configured a laptop with network share aliases on the desktop. While at the office, where the NAS drives are located, no issues; files opened and saved as quickly as expected. At home, though, where the network drives are not available, every time I would attempt to open or place a document the beachball of infinity would appear (and never release) -- I had to force quit every time. After much cranial exfoliation I realized the issue and placed the share aliases into a folder on the desktop. Even though the drives were not mounted and I made no attempt to mount them at home AI needed to touch those files and glitched when doing so. See if perhaps you have something like this going on in any of the directories you open/save from.
(2) There is a file, com.adobe.mediabrowser.plist, that resides at this location: Macintosh HD ▸ Users ▸ [current user name] ▸ Library ▸ Preferences -- it records a bunch of file information, and can get bloated. It is human readable, so you can open with a text editor and see all the accumulated info. AI creates this file if it isn't there, so you can delete it with no consequence. Instead, though, you might try quitting AI, opening the file in a text editor (not word processor) and deleting all the text in the file, then saving the file. Next, get info on the file via the Finder, and click the lock checkbox. Now, load AI and try to open/save. Google com.adobe.mediabrowser.plist; there are lots of words written on this file and how it can cause the exact problems you are talking about.
Hope this helps some!
G