As I said in my last post, you have to wait for the Arduino to get out of the bootloader after a serial init. It looks like you found that yours takes 1600ms.
Your VI should be structured to do the Serial Init only once at the beginning of the VI. Wait for 1600ms then you can write to the port as often as you like without any more delays. Just pass the VISA Resource Name along to each successive write. Have a look at the example Continuous Serial Write and Read.VI. Put your code in the Serial Write and Read while loop.