It's really important that you have vid-gpu for the entire chain. We've kept vid-cpu for legacy patches, but it shouldn't be used with high resolution video.
It get everything to use vid-gpu, you may need to disconnect the wires from the actors and reconnect them. In case you're not aware, many Isadora actor inputs/outputs are mutable, meaning they will adapt to the format of data passed to them.
Note that when this Zoomer actor is disconnected, it shows that the video inputs are mutable.
But once I connect a projector, the ports are no longer green indicating that they are not mutable any more. That's because, once you connect something, the format is chosen.
Anyway, glad that you sorted it out!
Best Wishes,
Mark