Hi Blitter,
No an External USB 3.0 Graphics does not always output 60Hz.
The Displaylink GPU insode does generate its own Video Signal, so the resolution support is depending on the GPU inside.
Having said that, they all can output 1080p at 60Hz also when your system cannot.
Not sure what screen runs on resolution 2560×2600 30Hz, but i am quite sure this resolution will not be supported also not in 60Hz.

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