Hi @Will,
Please make sure to turn off Videowall mode on your primary machine using the spacedesk Driver Console, then check if the performance improves.
In the meantime, there are also a few more things you can try to improve the performance with spacedesk. Please try the suggestions in the performance tuning chapter in the user manual https://forum.spacedesk.netuser-manual/#item_505.
Note that the hardware encoding feature, which will significantly improve spacedesk performance, will be available in the upcoming release.