Karma CPU is running much faster on my machine which is a M1 Macbook Pro, like a hella lot faster. I am used to sample settings as seen in Karma XPU however i.e 128, 250 etc.

The default in Karma CPU is 9 samples. Is this equivalent to 128 in Karma XPU and is the multiple the same as 250?

I understand however there is no exact science to sample settings and it will vary.

I think xpu sampling in 19.5 is just a uniform sample count identical for every pixel. Nothing clever about, it just runs whatever samples you specify over everything. I expect that to change in future versions.