As far as I understand that's not a very meaningful question b/c it depends on what CPU and what GPU. So it's a bit apples to oranges and depends on the user's configuration. There is a benchmark at the very bottom: https://openbenchmarking.org/test/pts/vkfft

Also, maybe a bit obvious.. but that even if there is no huge benefit - sending compute to the GPU frees up your CPU/application to do other things .. like keeping your application responsive :)