what I want to know is if there's any upper limit on the usable frequency (speed)?

Why? Andy, could you explain

Oleg,
for basic old fashioned multiplexing of a steady display with constant intensity, there may not be a reason to ask.

Now, let us have a look at something a bit more complex, but still keep it simpler than it is in reality for the sake of a 'brief' explanation.

Let us say you are making a display of RGB LEDs that shall show different colors and adjust the intensity to the ambient light.

1) it is known that if the display cycle is not complete in 10msec or less, the eye will discern a flicker.

2) a means of producing color at a constant intensity is to illuminate the 3 colors in 1/16 always keeping a total of 16/16.

now let us multiplex this display by doing one color at the time

so in 10 ms we need the following scans
1/16R
1/16R
2/16R
4/16R
8/16R

these 5 scans allow for any 16th of red to be displayed.

now we do the same for green and blue.

based on the above the 1/16 unblank time will be very short - go ahead and calculate.

NOW, in addition, visiualize that the ambient light is 1/10 of our max, so now we must shorten the unblank pulse to 1/10 of what we calculated above.

You will find that you will have to do with some VERY narrow pulses.

Erik