If you just mean that you're using Zotero syncing, that obviously shouldn't make Zotero slow.
What exactly is slow?
Can you disable any plugins and provide a Debug ID for Zotero startup through performing an operation that's slow, using the "Restart with Logging Enabled…" option?
What exactly is slow?
Can you disable any plugins and provide a Debug ID for Zotero startup through performing an operation that's slow, using the "Restart with Logging Enabled…" option?