Reply for @elextr:
Made some tests, the mean of "very slow" is almost 10 seconds.
the types of open a large txt-file: (1) Dragging and dropping a large file into the geany window. (2) Re-open all the opened files of the last session when start the geany. (3) Start the geany, open the 'open dialog', select file, click the 'Open' button. Select the filetype is "none" or auto-check(default).