Dell Optiplex 9020 Latency Issues

I have Dell Optiplex 9020 USFF (ultra small form factor) with a i5-4590S, on which I want to run Linux CNC with a Mesa ethernet card.


latency-test shows the following results while stress testing with six glxgears, firefox+youtube and firefox+surfing

1ms Thread : 20921ns jitter
25us Thread: 33322ns jitter

As far as I have read this should be sufficient for a mesa card. But latency-test also shows Unexpected real time error

Can this error be ignored? Is this unexpected real time error reflected in the jitter values or is so bad that is not logged anymore in jitter values?

Kernel 4.9.0-12-rt-am64

As much as possible turned off in the BIOS.

Bootparameters: quiet isolcpus=2,3 intel_idle.max_cstate=1 processor.max_csate=1 idle=poll usbcore.autosuspend=-1 mitigations=off nowatchdog nomce

Mitigations I turned off because I think this is useless for a CNC machine.
nowatchdog and nomce reduced the number of machine check exception and non maskable interrupts to zero.
usbcore.autosuspend=-1 because dmesg shows says that usb power management may be unreliable

Another thing that concerns me is that dmesg says ENERGY_PERF_BIAS: Set to 'normal' was 'performance'.
Is this a problem, if so how can I turn it off?

Is there anything else I could do to reduce latency?

The real time error is likely because you cannot run a 40 KHz base thread on that machine
a better test would be with only a servo thread:

latency-test 1ms 1ms
or
latency-histogram --nobase --sbinsize 250