I read your latest posts in both your threads (note that crossposting is deprecated or even forbidden on the VirtualBox forums, but you were lucky so far not to be caught by the moderators ), and have some remarks:
Virtual disk images change all the time when a VM is being used. Therefore, using a backup method like Time Machine was generally a bad idea, because the images would be backed up every hour then (assuming a long-running VM). This changed with macOS Big Sur, when Time Machine started using APFS itself.
APFS is known to be optimized for SSD storage and to be bad on HDD storage, that's why Apple first used it on SSDs only. You can find some background information on that in the corresponding Wikipedia article.
Extremely slow disk performance ubuntu linux guest Mac OS X 11.6.6
Re: Extremely slow disk performance ubuntu linux guest Mac OS X 11.6.6
I got a significant boost in performance after using vboxmanage clonemedia to create a copy of the vdi. This apparently defragments the block map for dynamically sized vdis during the copy process.
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This post contained the clue: viewtopic.php?t=8046