Then the app freezes during the work with the file - mainly, during cursor navigation. It freezes and unfreezes, freezes and unfreezes and so on. It seems like some heavy task is running from time to time in the UI thread.

Yes, it’s a huge file. Notepad++ has a lot of stuff it has to do while navigating around or editing a big file. Sorry.

As I said, the best option for you on this huge logfile seems to be Plugins > Big Files > Open BigFile, which will look at the file in chunks. If that doesn’t work for you, then there is no more advice I have for you with regard to Notepad++.

It depends on the amount of memory you have (physical, free physical, and swap size)

There are 32 GB of RAM and 2 TB of NVMe drive. There is a lot of free space.

Also, it depends on the contents of the file.

It is a log file from an app. There are a lot of relatively shot lines of human readable text.

and 15 minutes after trying to load, still not done

It’s not about loading, even a huge file loads pretty fast. Then the app freezes during the work with the file - mainly, during cursor navigation. It freezes and unfreezes, freezes and unfreezes and so on. It seems like some heavy task is running from time to time in the UI thread.

346 MB

Yes, like I said, Notepad++ is not optimized for huge files.
It depends on the amount of memory you have (physical, free physical, and swap size). Also, it depends on the contents of the file. I generated a series of 500MB of random alphanumeric/space bytes, with no newlines, and 15 minutes after trying to load, still not done; even a 100MB with the same no-newlines wouldn’t load within 5min. On the other hand, when I generated a file that had 4Million 100-byte lines with newlines (so 400MB), it opens reliably in about 30sec.

With the Big Files plugin, I was able to use the Plugins > Big Files > Open BigFile to open it (or its toolbar button), which breaks the file into chunks, and you can just edit a single chunk at a time. So if you don’t mind extra buttons to navigate through large chunks of your file, and edit those chunks piecemeal, then that’s probably the best idea for you to be able to stay inside Notepad++.

But Notepad++ is likely not going to be redesigned to make a truly seamless huge-file editor experience, because the editor component I.P. wasn’t designed for that, and it would require a complete rewrite of the application to use some other editor I.P. that handles huge files in a different manner; using a plugin like BigFiles is your best bet to stay within Notepad++.

the Language is set to None and -noPlugin doesn’t help.
The file size is for example 346 MB.

Debug info:

Notepad++ v8.1.2   (64-bit)
Build time : Jul 16 2021 - 01:59:50
Path : C:\Program Files\Notepad++\notepad++.exe
Command Line : -noPlugin
Admin mode : OFF
Local Conf mode : OFF
Cloud Config : OFF
OS Name : Windows 10 Pro (64-bit) 
OS Version : 2009
OS Build : 19043.1110
Current ANSI codepage : 1251
Plugins : none

I’m not sure if there are any experts in thread callstacks in this Community discussion forum.

Normally my first guesses for any “freeze” would be: (1) a plugin or (2) the huge file is using a syntax highlighter. To check (1), run with -noPlugin command-line option; to check (2), switch Language menu selection to None (Normal Text)

As always when asking for technical help in Notepad++, or trying to show a problem, giving the ?-menu’s Debug Info is a good idea. And giving more information about the file (actual size, rather than just saying “large”; file extension and what Language that Notepad++ auto-selects or that you manually selected; whether the file is on a HDD or SSD or on a network drive)

It should be noted that Notepad++ editor component hasn’t been optimized for huge files (on the order of hundreds of megabytes, and definitely not for gigabyte-and-larger files), because it tries to put the whole file into memory, and doesn’t do any memory-paging techniques. There are plugins like the BigFiles plugin (available through Plugins > Plugins Admin) that help with such things, but there are drawbacks to those plugin implementations.

But if you’ve found a true bug, you can read the FAQ about bug reports and follow the directions there to create a bug-report issue in the location it links you to.