There seems to be a few strange things going on here.
There are two separate apps that make up Atomic Scribbler: the Project Manager and the Scribbler app that works with your project (word processor, notes, etc.) The desktop icon leads to the Project Manager. This is a very light-weight app. It does nothing but list your recently opened projects and launches the second app when you select a project. The Project Manager isn’t doing enough to cause it to hang for any length of time when you try to open it. A problem here suggests a Windows problem of some kind: low spec or old machine with little available memory maybe.
If a particular project is taking a long time to load, this could be down to whichever scenes and notes were left open when you shut it down last. For a 300k word project, did one or more scene or note contain the body of those 300k words? And was that scene left open? Atomic doesn’t open any files on start unless they were previously open, so the size of a project should have no bearing on the speed of loading. The number and size of previously opened scenes and notes could have a bearing however.
A way to test this is to rename the project’s settings file in the individual project folder: “\Documents\Atomic Scribbler\PROJECT NAME\.atomic\settings.dat” Renaming this file will cause the project to open without opening its previously opened scenes. If it opens quickly, then the problem is in the previously opened content.
This sounds again like there’s a Windows problem going on somewhere. The Documents folder for a project is a plain folder with docx and rtf files, nothing more.