Google Sheet Script - Optimise a script that copies selected lines in sheet A and copies to last available line in sheet B

I have a Google sheets script that copies lines that have been flagged in sheet A and copies the line to the last available line in sheet B.

The script works OK when it goes through 10 to 20 lines but is EXTREMELY slow to go through a larger number of lines (target: 250 lines).

I need someone to optimise the code for the script to be faster (faster here just meaning ''normal'').

Thanks