Watch the splash screen as Photoshop loads, and see if you can spot it stalling at any point. If you do, note what process is loading at that point. If it is fonts, for instance, try checking them. There will usually be a tool available for what you need.
For fonts, as an example (with Windows)
Troubleshoot font problems | Windows (adobe.com)
Try launching Photoshop while holding down the Shift key. This prevents third party plugins and extensions from loading, so if the problem goes away I guess you'd need to uninstall all of them and reinstall one at a time.
What are your Scratch disk arrangements? Dodgy drives can slow things down dramatically. Try opening Preferences and clicking on the Scratch Disks tab. It should respond instantly. A dodgy drive might stall Photoshop for a full minute or more.
Do you have a zillion presets? Brushes, fonts etc.? They all need to load on start up.
But your best tool is the splash screen. You can't stop and start it, but if you have a screenshot tool like Greenshot, you can freeze the screen and grab an image. When Photoshop opens, use Ctrl n for new document the size of the clipboard, and paste your screen grab into it.
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