Sorry to hear about this @DanMcKie.
As this Help Center article (https://help.dropbox.com/accounts-billing/space-storage/file-storage-limit) mentions, the performance of the Dropbox desktop app can decline if you have more than 300,000 files synced to your computer. This is a soft limit and depends highly on the hardware specifications of the computer running the app.
Since you have a lot of files stored in Dropbox and you'd like to improve the performance of your computer, you can use selective sync to select only certain files to sync to your computer while you'll still be able to access all of your files from dropbox.com or other devices linked to your account.
Let me know how it goes!
PS:T his limit is related to the number of files, not to their size. Dropbox accounts with a large number of files may take longer to sync. A delay in syncing or a prolonged sync is not necessarily a sign of a problem. Adjusting your bandwidth settings may increase your sync speed.
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