10% cpu limit when running demanding task

this problem will probably make the computer scientists here chuckle but I try to run a ridge detection plugin on large tiff files and it takes for ever (read never finishing). when I crop it to 1/4 of the size, it does a reasonably fast job.

running fiji V1.52p
fiji has 25Gb RAM to use
images are roughly 5000x5000 pixels (I know this is not ideal and will look into compression later)

I notice that my Macbook pro is using only 10% of the CPU (and only half of the 12 cores) on an i9 intel processor. RAM usage is way below the 25GB. I this a problem of user allocation allowances (have not find if this is possible to restrict) on my computer or a physical issue with the image size?
I am plugged into the power socket.

any ideas in this great community to where the problem may be?

KR

Jan Soetaert