From your video it looks as if your printer does not use pulleys and belts, but uses standard threaded rods (M8?) for the X/Y linear movements...
If so, have you calculated the actual step rates Marlin must generate to reach your desired speed and compared this with the limits of your controller setup?
According to the table on the step rates page of the RapRap Wiki, using an M8 thread with Marlin on Mega/Ramps with 1/8 micro stepping is limited to 12mm/sec - which corresponds to about 6mm/sec with 1/16 micro stepping.
Try reducing the micro stepping. If that allows you to get to higher speeds, that might indeed be an indication that you are hitting the step rate limit of your controller...
If so, have you calculated the actual step rates Marlin must generate to reach your desired speed and compared this with the limits of your controller setup?
According to the table on the step rates page of the RapRap Wiki, using an M8 thread with Marlin on Mega/Ramps with 1/8 micro stepping is limited to 12mm/sec - which corresponds to about 6mm/sec with 1/16 micro stepping.
Try reducing the micro stepping. If that allows you to get to higher speeds, that might indeed be an indication that you are hitting the step rate limit of your controller...