Cubase 5.5.3 GUI lag

I am running into one of the many manifestations of Cubase’s epic GUI lags. It started out of nowhere yesterday. It occurs in only one project. Is it possible for the project file to become corrupted and create this issue?

Other environmental factors…

No updates, no changes. It only happens in the key editor window. I previously had this problem with a 15 track template with about 500 measures of midi note and cc data. The current problem I am having is with a template for a quartet–4 tracks and only 120 measures of midi notes and cc data. No track automation in project editor and very light plugins. I have been all over documented creation and have found no solutions or consistent diagnoses of this issue. This issue showed up on my older rig which had a different audio interface (presonus audiobox; ryzen 5 1400; 32 GB Ram 2100; GTX 1050ti; MSI A320M mobo). Any insights or suggestions welcome.

I am running:
Cubase 5.5.3
Windows 10_64 Home
Ryzen 7 2700x 8core
16GB Ram 3200
Asus Prime Pro x450
SSD for OS and Cubase
SSD for project files and VST libraries
Vienna Ensemble 6
GTX 1070ti with latest driver: no on-board GPU
Presonus Firepod 1394 controller running in Windows 7 Compatability