Where can I find the "restorecount" value macOS uses to track open windows and reset it?
The end value is not the number of windows you have opened, it is a Cocoa timestamp. 570214618 corresponds to January 26, 2019 4:56:58 PM GMT. As far as I can tell, the purpose of restorecount.plist is just to track how long it takes to restore the windows.

For apps that are taking a long time to start, you can try cleaning out their saved application state and see if that helps, but there are lots of other reasons an app might take a long time to start. The most common reason an app takes a long time to start is that it needs a lot of memory and you are already using all the available RAM so some memory needs to be compressed or written to disk in order to free up memory for the app. You can get a sense of this by looking at the "memory pressure" in the Activity Monitor.

You can also look at the amount of Swap Used (also in Activity Monitor). If that number goes up when you launch an app, then not having enough free memory is definitely a source of delay.