Performance problems in 2.9

I understand that 500 ms is practically unusable.

But is there any significant reduction in CPU usage?

If so, I'd try to comment some functions here:
github.com/LinuxCNC/linuxcnc/blob/c0c4e9...ccapy.py#L2384-L2416
And I would find out what it does.

The CPU reduces but not quite as expected - eg 250 will reduce to 25-30% CPU but 500 only reduces to 20%.

I can certainly comment out some of the functions (I just have to find them on my local system! Hints appreciated.).

Many thanks

You need to learn how to RIP installations:
linuxcnc.org/docs/2.9/html/code/building-linuxcnc.html

That may well be true but I couldn't possibly comment!

My lovely NUC decided to corrupt its BIOS after I went to look to see if I could under clock it (I suspect a power glitch at the most inopportune moment), so now having to spend some time rebuilding on another NUC - this time with a fan!

It is the exact same issue for me. Good performance in 2.8.4 on Debian 10. Unusable in 2.9 on Debian 12. Granted my pc is not that powerful. But still something seems to be off.

Edit: Okay found the solution: Replaced the stdglue.py from my 2.8 configuration with the stdglue.py from the 2.9 gmoccapy sim configuration within the python folder of my machine configuration. Now it works like a charm. No lags and the performance is as it used to be.