I've got an Intel NUC D54250WYK and looking to get an 34" 3440x1440 screen.
However the maximum supported resolution seems to be 3200x2000 @60 Hz or 3840x2160@30Hz.
How much will that effect the performance? Is 30Hz unusable? Should I get an 27" 2560x1440 instead?
The maximum resolution supported on this NUC is
If you're trying to run a 27" 2560x1440 it should work fine.
For further reference go to Intel® Core™ i5-4250U Processor (3M Cache, up to 2.60 GHz) Specifications.