I have a feeling that this is partly because of the Undo capability of krita along with another situation with RAM usage that I’ve started to look at.
(I intend to raise a topic about it soon before going into formal bug reporting mode.)
Can you run your Task Manager as you’re doing your mosaic adjustment work and regularly check how much RAM that krita is using?
Are you following the suggestion by @wojtryb of using file layers or are you loading the images as paint layers inside krita?
As a possible help, but not a definite cure, can you go to
Settings -> Configure Krita -> General group -> Miscellanaeous tab
and set the Undo stack size: to zero., then press the OK button.
Also, in Settings ->Configure Krita -> Performance group -> General tab,
set your RAM memory limit to 13GB (don’t have any other applications running at the same time) and your Swap File Size limit to 6GB (assuming you have enough space on your hard drive).
Since all you’re doing is moving images around, the loss of Undo capability won’t be a big problem for you. As I said, it may help a bit.
If the RAM usage starts to get too big, as noted by the Task Manager, you can Save your work so far as a .kra file, close krita and then restart it and Open your .kra file to do more work on it.