I recently began testing factorials using an i10-10900k and it takes very long to process a number. It's been 34 hours and I'm at approximately 61%. I remember a test took about 24-30 hours on i5 and i3 cpus (Windows 10).
It's a 10 core and I run 10 tests. L3 cache is 20 mb which is not ideal for 10 tasks but more per task than on these other machines.
The main difference is that the i10 is running under Linux/Ubuntu. Could that be the problem?
There is nothing else running, according to htop the CPUs are fully utilized and everything else works fine. I did a lot of factoring using that computer and speed was as expected.
1281979 * 2^485014 + 1 is prime ... no further hits up to: n = 5,700,000