I've run into the same problem.
I turned to the support team and they proved helpful.
Their tips helped me solve my issue (at least as of now) and my unit is working properly now, without having to uninstall universal search because, as they say, Synology needs that to work correctly.
My problem was stemming from the indexing of all the metadata in my 30,000+ digital negatives. I'm copying their tips for your benefit.
Hello dear customer,
Thank you for reaching out to our support and providing details about your indexing issue.
I understand that you want to completely disable the indexing feature on your NAS; however, it's important to note that completely disabling indexing might affect the functionality of Synology Drive and not work properly, as it relies on indexing to synchronize files.
That said, I recommend checking if the indexing is taking time because it is also indexing the content of the files, such as the metadata of the photos. This could explain why the process seems to be ongoing. In such cases, please be sure here is unselected:
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You can stop and restart the SynologyDrive package to get rid of loops of indexing: enter image description here
You can also pause the indexing, if you need the resources to do something else and let the indexing run only overnight.
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I have found several times their support team to be very helpful and extremely responsive. Disabling Universal Search doesn't sound like a good idea at all.