Hi avishay, Welcome to Yamaha Synth!

If you require to play it like a piano, you should look into actions that mimick the acoustic grand piano. This is why there are weighted action versions. Reality is that as you pay more for keyboard actions, they do actually get better... Piano actions like on the Yamaha CP4 Stage will give the most grand piano-like response in this area... Not only are the white keys natural wood, the key tops are the same Ivorite found on acoustic pianos, but there are three sensors detecting key movement per key. The triple sensors allow the ultimate in quick key response... The key does not have to fully return in order to retrigger... So the CP4 Stage is most like an acoustic grand piano in this respect... At best "synth actions" emulate more of an upright grand action when it comes to key speed. The key must return a good amount before you can initiate a new note-on event with that same key.

BTW, It's not the 16th notes, it's the tempo. But we know what you meant. You don't mention what specifically you are doing ... If recording, simple slow the tempo, if performing live, recognize that same key repeated 64th notes at 300bpm probably will not happen, find at what tempo you find the problem... Then simply don't play repeated notes faster than your instrument can reproduce them. You wouldn't expect to do everything you can on a solid body slim neck electric guitar on a wide neck classical guitar. Play your instrument within its given parameters. You cannot perform certain piano pieces on an upright, that you can on a grand action... Gravity is your friend in the acoustic grand, and not so much in an upright action.

Solution, play your MOXF6 via MIDI from a CP4 Stage. While perhaps not the most practical of solutions, it does solve the speed of key return issue... So if money is no object... Go for it. Alternate solution, the MOXF comes in an 88-key Graded Hammer weighted action, and while not the NW-GH3 action of the CP4 Stage, it certainly plays more piano-like than the synth action of the MOXF6.

If you are playing from a MOXF8, make sure you adjust the VELOCITY CURVE to match your particular playing style. It can dramatically affect your perception of the tone engine response.
Press [UTILITY]
Press [F1] GENERAL
Press [SF3] KBD (Keyboard)
Vel Curve (Velocity Curve) set the curve to the "Soft" for a feel that is easier to the touch. But try the different settings (10-15 minutes each) and see which one makes you most comfortable
Press [STORE] to write your preference to your instrument's internal Flash ROM

Hope that helps.