Hi piotreba ,
GREP Styles can slow down InDesign text composition tremendously.
The problem of the OP here is with plain text without any special formatting.
I think, that bug was fixed in the meantime.
Did a test with CC 2019 version 14.0.1.209.
Had a 300 pages document, text in one story with only the basic paragraph style applied.
"Select All" of the text was instantly selecting all the text. No delay. Could scroll through the pages with the text selected.
Exported to IDML, opened the document with CC 2018.1 and was seeing a huge delay after a "Select All" on the text. It took CC 2018.1 about 10 seconds to revert to normal behavior so that scrolling through the pages was possible.
Regards,
Uwe