My plan is for testing purposes only is to add a ruler to the background so I can measure exact pixels. Then change the visual feedback icon so it updates when the button can be pressed instead of when it is pressed. I can then take a video of me playing the game like this, still pressing A (doing so adds about 10 pixels of lag from start to finish). Then I can go thru that video and see where exactly I need to place the notes.
I also plan to have the camera only move 4 bars at a time (currently it just runs to the end of the song). Every 4th bar ends in a half note, so I’ll have time to then have the camera jump to a new position and start moving again so that the next 4 bars have better accuracy in case you messed up on the previous 4 bars. This should help a lot with the third section where you have the sets of eighth notes as they create a ton of lag.
Of course I should note that all of this will only work properly on either an actual Game Boy or an FPGA device like the Analogue Pocket. Emulators simply can’t match the lag of original hardware, and everyone of them is different.