Noticed today when modifying a document (C++ < 2000 lines) that hadn't been used for a while:
1. type return to get a new line 2. blank line is created and previous next line is moved down but the rest of the screen does not move 3. cause an update, eg type a char or move the cursor to a different line and the rest of the display updates
Only happens once per "stale" buffer, after that all additions are fast. System is not swapping or anything that should make a buffer that hasn't been shown for a while slow (AFAICT).
X11 Linux Mint 18.3 GTK 3.18.9, GLib 2.48.2.