Upgrade StorageNo warning that storage was full
I had various issues with Zotero, e.g. very slow Word plugin and generally slow performance in the app, so I delinked and relinked the account to the app, after which lots of my records were missing. Turns out my subscription had expired in March so I was over the storage limit, but I didn't get a warning of this. You might want to check this isn't a general problem.
Unlinking your account and deleting data isn't something we recommend as a troubleshooting step, and it certainly wouldn't have anything to do with performance.
Being over your storage limit has nothing to do with items in your database. Zotero has unlimited data syncing, with a quota solely for file syncing. So if recent items are missing online, you likely had auto-sync disabled, or you were getting some sort of sync error and didn't resolve it. Either way you'd get a warning — Zotero shows a warning if you haven't synced in a while due to auto-sync being disabled, and if you're getting a sync error it shows an error icon next to the sync icon in the toolbar that you can click on for more info.
If you hit your storage quota specifically, you'd also get a warning in the toolbar, but if you were getting an actual sync error that would take precedence, and the error icon for that would appear instead.
In any case, if you have a backup of your Zotero data directory from before you unlinked your account and told it to delete your data, you could restore from that.