Slower HDR merge on new Mac

I am currently using a Mac Pro 4.1 upgrade to 5.1 with AMD RX580 and merging 5 camera shots to HDR which takes 20 seconds.

My new Mac which I assumed would be quicker with intel i9-9900k, amd 5700XT gpu and m.2 drive.

But it takes twice as long at 41 seconds.

Using Benchmark 5 its quicker than the old MacPro 5,1 in all tests.

I turned the gpu acceleration off in the Camera Raw pereferences and it took the same 41 seconds

Does anybody know if the the HDR merge uses the gpu or cpu?

Or why an old MacPro 5.1 would be quicker?