When my project has twenty *.ino files with some of 1000 lines per file, then they open slow. Meanwhile, I can do other things (so I thought), but what I type will get into a *.ino file.
Ubuntu linux, Arduino IDE 2.0.0-beta.3
I opened a project for the first time. The board was not set. I opened the drop-down in the bar for "Select Other Board & Port", then I searched for "uno". However, the tabs with *.ino files were still filling and the typed text was stored in a *.ino file.
This slow opening happened just once. Even after restarting my computer, loading the *.ino files is still many times faster.
Because the compiler gave an error, I could find the text that got into a *.ino file.
The dot in the tab label (after the file name) disappears so fast, that does not help to find a changed *.ino file.