Photos for Mac eating all CPU cycles.

I've been having this problem intermittently, but recently it's been happening al the time. On Monday I fired up Photos, and it caught up with the iCloud ones and then proceeded to take 180% of my 4CPUs. Since then, it's been pegged there constantly for over 4 days.

I have this synced to iCloud photos, and its the system library. Doing a 'fs_usage' on Photos gives a lot of output like the list below, with the entries under derivatives, slowly changing.

( I've removed the last 2 columns from fs_usage to make it a bit easier to follow).

Any ideas of what's happening, or how to stop it. I'd like to be able to work on the iMac without the fans on all of the time, to be honest....

BTW, Photos on my other Macs don't have this behaviour - Big Sur and High Sierra. This is running latest patched Mojave.

14:26:50.758441  getattrlist                            /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/proxies/derivatives/c3/00/c331/UNADJUSTEDNONRAW_thumb_c331.jpg
14:26:50.758544  open              F=14       (R_____)  /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/proxies/derivatives/c3/00/c331/UNADJUSTEDNONRAW_thumb_c331.jpg
14:26:50.758546  fstat64           F=14
14:26:50.758558  statfs64                               /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/proxies/derivatives/c3/00/c331/UNADJUSTEDNONRAW_thumb_c331.jpg
14:26:50.758595  pread             F=14   B=0x2eda0    O=0x00000000
14:26:50.760814  lstat64                                /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments/ThumbJPGSegment_22.data
14:26:50.760830  stat64                                 /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments/ThumbJPGSegment_22.data
14:26:50.760840  stat64                                 /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments
14:26:50.760879  pread             F=10   B=0x4        O=0x0091b000
14:26:50.760882  pread             F=10   B=0x1d53     O=0x0091b004
14:26:50.762010  lstat64                                /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments/ThumbJPGSegment_22.data
14:26:50.762025  stat64                                 /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments/ThumbJPGSegment_22.data
14:26:50.762035  stat64                                 /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments
14:26:50.762056  pread             F=10   B=0x4        O=0x0091d000
14:26:50.762058  pread             F=10   B=0x1f3c     O=0x0091d004
14:26:50.762251  pread             F=14   B=0xeb3      O=0x0002deed
14:26:50.763239  lstat64                                /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments/ThumbJPGSegment_22.data
14:26:50.763254  stat64                                 /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments/ThumbJPGSegment_22.data
14:26:50.763264  stat64                                 /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments
14:26:50.763301  pread             F=10   B=0x4        O=0x0091f000
14:26:50.763304  pread             F=10   B=0x1f62     O=0x0091f004
14:26:50.764758  lstat64                                /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments/ThumbJPGSegment_22.data
14:26:50.764876  stat64                                 /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments/ThumbJPGSegment_22.data
14:26:50.764900  stat64                                 /Users/MYNAME/Pictures/Photos Library.photoslibrary/resources/segments