is it a limit of the program or mistake something?
Yes, there is a limit in the concept. vvvv organizes everything in individual frames. You can do only one operation in one frame, which means you van change the value of the port only once in a frame - so you are limited in the maximum speed in which you can toggle your port bits. we may change the port object to allow for some programmed bit sequences at one time, but this is currently not on the list.
If you dont have any complex rendering needs it might help to set the Presentation Interval pin in the Renderer (EX9) to Immediate, and use MainLoop to set the framerate to something more than the default 60fps.
Anyway even if vvvv would allow doing this, Windows is not a realtime operating system and you will have difficulty doing clean pulses of several hundred Hertz alone in software.
So you might be better investing in an arduino board and doing the timing of the pulses there. The HowTo Motors wiki page gives some more tricks.