I don't know the udmx interface, but you should keep in mind that it is possible that the interface hardware can't keep up. Similar problems arise with a Velleman VM116/K8062 DMX interface, which is why no one should by it.
You need to rule out the possibilties:
1. While using the udmx on FreeStyler, have a look at the DMX output screen in FreeStyler. Notice how fast the values are changing. Are they changing fast as it should, or just as slow how it should not?
2. Configure FreeStyler to use a virtual interface. Nothing comes out your udmx then, again have a look at the dmx output screen and notice how fast the values are changing now.
3. Test the udmx interface on other software than FreeStyler, let it control the same amount of channels with about the same chases.
4. As a test, do not use Sun- or Ledtrix but create a sequence that does about the same chase. Is that also to slow, or good.
If you ask me to guess, I'd say the udmx has the same problem as the Velleman VM116/K8062. Its 'onboard' processor is badly choosen, or the firmware is written badly. Therefore it gets to slow when more DMX channels rise above zero. But this would just be a complete idiot guess of me.
Anyway, very simple way of testing this:
1. Attach any fixture, say you do that on DMX channel 1. Make an override button that flashes the fixture. (so that you press it, it
immediately goes on, not half a second later).
2. If the above point is set up, you need to find a way of setting ALL remaining DMX channels to a value higher than 1. My way: add many, many, many PAR64-6 BARs until all DMX channels are filled. Select them all, and slide the channel values 1 to 6 to 255.
3. Press the override button described in 1. Try several times. Does it go on immediately?
If no: It might take at least more than 500 msec for the fixture to go on. Congratulations, your interface has the same 'shitty' problem as the Velleman VM116/K8062.
If yes: It seems the problem is not the interface, but it stil might be. And it can easily be anything else on your computer.
) the refreshrate is okay. When you use the Plugin for more than approximately 20 LEDs (20 Adresses) it gets very slow. On a big 20x12 Wall the Refreshrate is araound 0,5 Hz - 1 Hz. 
).