I have two documents open. The primary doc is sort of big - 1.3GB - and the secondary doc isn't exactly small - 895MB - but I routinely work on much bigger files and have never had the issue until the 2019 update.
When I use the tabs at the top of the work area to switch from the smaller doc to the larger main doc it slowly rebuilds histograms every time. Even when nothing has been done since the last switch - just the act of switching triggers the wait bar. It's a huge momentum killer since I pop back and forth between multiple documents constantly. One note: The main doc that triggers the rebuilding was created in CC 2019, while the other doc was created in CC 2018. Switching from the 2018 doc to the 2019 doc triggers the rebuild, while switching from the 2019 doc to the 2018 doc does not.