I missed that indication in the status bar introduced with UE v20.00 also quite long. But now I could see it with UE v20.00.0.1052.
In prompt area on left side of the status bar at bottom of the UltraEdit main window Parsing x% is displayed after loading a binary or text file and scrolling down the first time for example with key Page Down.
The initial display with first few KB of a large file is quickly displayed if the large file is opened without using a temporary file. The parsing indication occurs next on scrolling down in the file.
But if a large file is opened with usage of a temporary file, UltraEdit first creates a copy in %TEMP% without any indication during this copy process. The document window simply shows nothing until the copy process finished. Then the first lines (text file) or data (binary file) are displayed in the document window. Scrolling now down results in starting the parsing process indicated in the status bar.
I don't know why UltraEdit parses the file. I assume, it is parsing for line terminators.
I don't see a parsing indication with configuration setting Disable line numbers at Advanced - Configuration - Editor Display - Miscellaneous being enabled. With this setting checked as recommended for working on large files, scrolling quickly downwards with key Page Down rapidly pressed or hold, or using the vertical scroll bar results in displaying quickly the file content without any blocking by a parsing process up to some point in the file. Then the document window shows no more lines. After waiting a few seconds, scrolling can be continued with appropriate display updates.
So it looks like a file parsing process takes also place with Disable line numbers configuration setting being enabled, although that should not be the case, and just starts later. I need to further analyze this behavior using Process Monitor from Sysinternals before I can report my observations to IDM support by email.