With this patch grid/column select all time is reduce to ~1 second from 8-10 seconds.
This benchmarking is performed with 100k rows and 2 columns (int, text).
The solution was to use simple array concatenation instead of underscore union while getting index of all selected complete rows.
Underscore union function is only useful when user selects different ranges from grid and those ranges overlaps. In this case union function removes duplicate (overlapped) rows.
However result grid in sqleditor do not support overlapped row selection so we can simply cancat rows from different ranges without worrying about overlapped row selection.