The lag is due to the GPU being fully utilized for FAH and there not being interrupts for the GPU. Once the GPU works through its compute buffer it can update the screen. Unlike the CPU which has interrupts for things like the keyboard to reduce user input lag. Its a sign of FAH getting the most out of your hardware.
Core18 units tend to have lower GPU util, like 1-3% or so, which is enough to reduce screen update lag. Best thing would be to run the monitor off an iGPU or a card not crunching.
If there is a driver version thats better then its probably worse for PPD.
There are other BOINC projects much worse for lag than FAH btw. Collatz when configured can make a computer VERY frustrating to use to do anything at all. Asteroids can peg GPUs at 99-100% and make any video require a pause.