Summary for anyone else following this: if you have the laggy scrolling bug, please try the following two builds:
Build 3:
https://scrivener.s3.amazonaws.com/beta/Scrivener_3_45_UIValidationTest.zip
Build 4:
https://scrivener.s3.amazonaws.com/beta/Scrivener_No_WindowUpdates.zip
And let me know how scrolling is in each.
Thanks!