Sorry, you can’t make vvvv run any faster than, I don’t know 120 frames/sec, so that means only 2 updates per sec of your grid at best.
I am pretty sure the arduino has some code somehwere to convert the values it receives from the serial (a string) to a nicely filled array of 64 int elements.
Do you use firmata? (dunno what the current status is on that)
My idea would be to ask the Arduino people (they are almost as nice as the vvvv community ;) ) how you can convert a Serial.read() string that is 64 integers (0 or 1) to something you can feed your LED array with.
Without a code a can’t pinpoint and solve the problem, but I do understand it.
Good luck :)
edit: I found this on the Arduino Forums:
To convert a char to an int, subtract ‘0’ from the char:
int S = SlotNo - ‘0’;
edit2: Send me a PM so we can talk on Skype/MSN about this, I just got my Arduino Mini, and I can use all the help I can get. (building a 1 channel MIDI device, nothing to do with vvvv)