That depends on your machine and your raw files.
You can measure it by enabling the measure checkbox and running RT from command line (to get the measures).
Good way to measure ist to put a file 7 times into the queue and take the median of the 7 measures. Then repeat the procedure for different Tiles per thread
I measured up to 20% speedup for RCD demosaic on my machine when switching between 1 and 6 tiles per thread