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Sorry to hear you’re facing problems 
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Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): Nextcloud Desktop Client Version 3.9.0 (macOS)
Operating system and version (eg, Ubuntu 20.04): macOS Mojave 10.14.6
Apache or nginx version (eg, Apache 2.4.25): replace me
PHP version (eg, 7.4): replace me
The issue you are facing: File and folder access is very sloooooow
Is this the first time you’ve seen this error? Y: Problems ever since using a new disk on my Macbook AIr. Previously not a problem on same Mac with different disk; both SSD.
When using Finder or using Files Save As etc in Applications, I have to wait sometimes minutes to get a response. Also simialr when opening/saving files/folders in other applications (eg LibreOffice). So working with Synced files/folders is incredibly inefficient. I don’t have this problem with Synced files on Dropbox, and didn’t used to have this problem with Synced files from same Nextcloud location, but working with previous set-up (different disk, probably earlier macOS vesion but not sure what version it was).
Thanks, Colin