I'm interested in this as well. I have spent many hours trying to get > 11-12 m/min smooth motion on a large format router without joint following errors and/or jerky motion. Using the old control or TwinCAT CNC with the same servo driver config works fine at 30 m/min.
I'm no expert on LCNC internals but if RTAPI_TASK_PLL_SUPPORT is defined in linuxcnc-ethercat lcec_main.c then it appears that syncronization is attempted between the ethercat application cycle and LCNC with a max error of one app period...maybe this is where Hakan is getting the potential extra cycle delay.
Since you have the ethercat master running outside LCNC it seems like it would be difficult for those to be perfectly syncronized but perhaps that does not matter in application. I know TwinCAT is running in hard realtime on one or more dedicated cpu cores outside of operating system and with their tools you can see how much time is being used in each cycle. It would be nice if there were a tool for LCNC where you could see that info but I'm not sure it's even possible in the current architecture.
Be careful because it is configurable for twincat Nc task axis setup is axis lag watching possible to disable and there are any running speed possible to get... I think they have 7phase generator to control to axis movement which is probably more powerfull then lcnc trajectory generator ... They have native ec drivers for they native hardware .. but I am not specialist, I have just 15y of experiences which is nothing...
I know just about the single cycle lagging but there is possibility of having the DC timing problem which can cause the f error during high speed .. therefore is great practice to take DC config of slave take directly from the TC3...
I have problems with ferror too but they were caused by servo setup, dc config and slave timing setup... I have ran at 250us well 4axis
