Yes, this is normal. For the same material, the computational time is proportional to the square of the crystallite size, so, the computations for 21 nm crystallite will take 12.25 times longer than for 6 nm crystallite. However, the atomic density also varies from one material to another. There are also other parameters that may affect the computational time.

See the correct answer in the next message.

Sorry, I wrote my answer at 3 am local time, and it is terribly wrong. The time, t, required to compute the powder diffraction pattern of a crystallite is proportional to the square of the number of atoms in it, N. N is proportional to the cube of the crystallite size, D. So, t ~ D⁶. So, the computation of powder diffraction pattern for 21 nm crystallite takes 1838 longer than the computation for 6 nm crystallite (if the atomic density is the same). However, for any crystallite, the app needs to compute the atomic ensemble, print the files, etc. That’s why the lowest computation time is always a second or so.
In the future I'll improve the scheduler, so the initial data for the small crystallites will be sent only to the CPUs.

i found that you need to reserve one whole CPU core otherwise it can take an age.

Tasks on my 7950 were taking 5 mins or so compared to less than 10 seconds on similar GPUs. Once i made sure it had a full core, times dropped to a few seconds.

Good idea.
If you send too much little wus to gpu, you risks a DOS of scheduler because of the intense upload/download. Milkyway@Home, recently, has increased 5 times the data in gpu app to reduce the server load.

Other "gpu projects" need a dedicated cpu core.
App_info.xml solves the problem

i found that you need to reserve one whole CPU core otherwise it can take an age.