Preemptive RT on Jessie or Stretch?

Yes. Old steam machines are used 100 years ago. Old cpu's are used 10 year's ago. If you want to use old cpu, i think you have to compile linuxcnc 1.0
I hope github will be able to go back to this version. Linux is just like an old arcade game. You can go back in time.

Well, this is not the point: LinuxCNC 2.8 simulation and kernel 4.xxx work perfectly on these older systems. RTAI Kernel work quite ok. What does not work is the PREMPT-RT kernels. I just got bitten by the same effect on machinekit+BBB: xenomai kernel works like a charm, preempt-rt does not (0.5s latency .. that's something to be beaten).

In the meantime I made a bit of a progress: This latency-histogram from the latest linuxcnc build from git on devuan ascii. It's running on a T60 (ATI) with coreboot, NMI and SMI disabled, vesa graphic driver, modem + wlan removed. The only thing I'm not happy with is the old kernel from wheezy