I may have fixed it with the advice from REVIT Instantly crashes when opening Dynamo - #3 by jacob.small
I deleted %appdata%/dynamo/Dynamo Revit/2.16/DynamoSettings.xml
Then Dynamo started quickly. Had to re-do Package paths, etc.
Could this have been the culprit? Looking at a recent dynamoLog_....txt:
2023-07-27 15:24:13Z : The folder ‘C:\Program Files\Autodesk\Revit 2023\AddIns\DynamoForRevit\Built-In Packages\Packages’ does not exist
2023-07-27 15:24:13Z : Dynamo will use the package manager server at : https://www.dynamopackages.com