You're in a group with over 200,000 items (including child items). That's likely the problem here (and the speed may be affected by whether that library is selected at startup). The next major version of Zotero will hopefully be able to speed up database access quite a bit with the switch to a new architecture, but right now loading that many items unfortunately can just take a long time.

If you can provide a Debug ID for the startup using the "Restart with Logging Enabled…" option, though, we can see exactly where it's slow for you and if there's anything we can do in the meantime.