After running many diagnostics, we’ve finally discovered the cause of the slow score editor. The Windows Fault-Tolerant Heap had been enabled for Cubase14.exe, maybe as a result of crashes in an earlier Cubase 14 version. This is a special layer in the OS that adds more memory protection for certain types of crashes. It also makes memory requests much slower, which the Scoring Engine is sensitive to.
I asked Mads to take a look in the Application Compatibility Settings to see if Cubase14 was in there and it was:
I found Cubase 14.exe in HKEY_CURRENT_USER_Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers and deleted it + stopped the FTH. Now the score editor works almost as fast as the key editor!