A Delta printer requires constant complex calculations to produce straight lines while printing. The firmware, therefore, spends most of its time figuring out the step and timing sequence, and only the little remaining time between interrupts and these calculations is given to the user interface.
Marlin doesn't document any configuration parameters that would improve the user interface responsiveness, and in any case such improvement must necessarily come at the cost of printing speed and/or accuracy. The processor is being used to its maximum capacity. The only thing you might be able to do is dig into the firmware and try to change it yourself, as this is not a clear option within Marlin.
If working with the user interface while printing is important to you, your next step should probably be to change to a faster 32 bit processor. There are a few firmwares available for ARM and other architectures which may resolve your situation.