Dev here - I agree it is too slow - really needs a bit of machine code for the fill routine, but this is a speccy jam game release where only Basic is allowed so have to suffer the slowness i am afraid. I have added a little code to try to fast fill from the top when a full line is a complete colour just to increase the speed a bit (not perfect but does the job)
What to do - choose a colour and flood fill from top left. Fill will continue right until it hits a colour that is different from the one currently filling over or itself. If it hits its same colour, it will continue. Breaking into a new line is only achievable by choosing the colour in the left column. Try to determine which colour will give you either a new line or the largest flood fill to try to complete the whole board in one colour in the least amount of moves.
Thanks for the positive feedback so far.