Zotero 7.0 is slower than 6.0
Currently, the issue with Zotero 7.0 is that it lags when opening related PDF files and browsing through the files. The preview of PDFs also suffers from this lag. The performance is not as smooth as Zotero 6.0. Hoping for an improvement in this regard.
There was a regression in recent betas that could cause things to slow down while files were syncing. That should be fixed now in beta 78.
If you're still seeing this in beta 78, you'd want to test in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which disables all plugins. If you're still having trouble after that, we'd need more details on what exactly is slow.