CosmicR (./19) :
Just looking at the code you posted - you can speed it up further by setting the timer to skip over the first N lines of the screen - until the effect starts. There is no need to have timer interrupts happen at all on lines where there is no 'effect'. They are quite slow in themselves

Yes I tried to do that, but I wasn't success. Changing the speed 2 times in 1 frame update didn't work. Maybe did I something wrong, or maybe there is a proper way to do it ? Playing with the LSPC is a kind of alchemy, especially though C code., I hope to figure out more things about it in the future.