My Zotero becomes very slow when the number of references reaches 287. Even after reinstalling, the issue persists. When flipping through PDFs, it often gets stuck with a spinning circle and shows "Loading...". It's really frustrating.
Not sure what you mean by "the number of references reaches 287".
In any case, Restart in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins. If that fixes it, you'll need to identify the plugin that's causing it and then report the problem to the plugin's developer. If not, we'd want to see a Debug ID for Zotero startup through reproducing the problem, using the "Restart with Logging Enabled…" option.
Upgrade Storage
In any case, Restart in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins. If that fixes it, you'll need to identify the plugin that's causing it and then report the problem to the plugin's developer. If not, we'd want to see a Debug ID for Zotero startup through reproducing the problem, using the "Restart with Logging Enabled…" option.