Ever since upgrading from the 2015 CC release of Photoshop two years ago, I've had issues with the new file dialog box. It frequently appears as a blank gray box containing no options, and fills in after 5-10 seconds. Other times, it doesn't fill in at all, and I have to close it and try again. This has persisted through OS upgrades (currently on Sierra) and CC upgrades.
One of the things you can try is return to the old style New Document dialog box.
I realise that doesn't quite answer the question, but will hopefully help speed things up again for you.