PiDiCNC Control System

Does it have an option to set the amount of video memory in BIOS? Linux in general is very good at utilising memory, so it makes a huge diference if there is plenty of it.
Most desktops with integrated graphic cards have that option, that would leave more memory to video card even when Linux is running out of it. Would make the overal performance of Linux slower but chances are the update of the video would be a bit faster.
I do not have any of the RPI so i can not test this, sorry bro.
There is another posibility to test, but requires another computer. SSH from the other computer and start Linuxcnc, that way the GUI will be on the other computer.
Did you check TOP to see how and what process is slowing things down?

P.S. Sorry for the Polish, my bad. You are in Slovakia, or as i like to call it the country with most beautiful women on earth. Been in Bratislava for 3 weeks, came back with some minor PTSD.

You are right about always liking what is not yours, but i do know why i insist on that, i have traveled half of Europe and Balkans and Asia (just Turkey).
Back on topic,
The general idea that that type of computer ( RPI, BBB, UDOO etc ) are very powerfull falls flat on it's face when you try to run anything that does a lot of procesing and heavier graphic, they just can not perform as we would expect them.
Case in point, i own a Galaxy Note 3, it is still a monster as far as smartphones go ( 4X2.3 GHz processors overclocable to up to 3 GHz ), for all intents and purposes a very fast machine......that is until you install Linux on it, it just can not cope with it, and i am fully aware that it does not run natively, but i can run virtual machines on a 2X2.53GHz laptop quite nicely.
Not going into the diferences between architectures and how they process things, but they definitely are not the same animal ( nature: house cats are in the same family with Lions and Tigers, although the latter are about 200 times biger and stronger).
After all this, i still like those smal pieces of c....., they do have a purpose and do an astonishing job, for some specific needs ( we had a lot of power cuts till 3 years ago, so having things battery powered was a must ), they use around 10W of power at full throtle, that Laptop i mention goes to 48W under full load, had one that used 69W ( quad i7 ).
When it comes to machines, power saving is not important much, my Thermal Dynamic plasma uses 28KVA at full throtle so sawing 30W is useless.
That is the main reason i gave up on smal computers since the time of RPI 1.
regards,
Tom

One more thing, linuxcnc has 3 diferent timings for updating it's own things, they are in ini file and usualy set at 0.010, try seting them at 0.1 and give it a run, see how it impacts the refresh and performance, also set the base thread as high as you can posibly go, try 100000 and above up to 500000.
I am sure this helps, i just can not recal what i did a few years back to make it run on a 500MHz processor.
The base thread will limit the step rate, but if using lower microsteping it should be just fine.

Well did some testing based on you info, so here are the results:
Gcode file with 48000 lines > opens in a second and runs perfectly, screen updates a it slower at a rate of 3 fps roughly, linuxcnc usable stop/pause/start work properly.
Gcode file with 146000 lines > opens in 5 seconds, i can pres start and it works without a hitch, but linuxcnc is unresponsive totaly, when presing start there is no change in the button or anything on linuxcnc gui, presing stop also does not show but after 20-30 seconds it stops, trying to open another file after this also takes over 30 seconds. During this test i also noticed on the system monitor that the memory usage fluctuates a lot ( 3GB of RAM ), while TOP shows Axis as the main consumer of memory and processor ( over 125 constantly ).
During all this computer is usable and responsive as usual, everything else works properly and snappy.
Have to test tomorrow with an older version of linux and linuxcnc, just to see if the old trajectory planer with low look ahead works for this.
All in all a very starnge behaviour for sure as i can not think of a reason for this happening. After all it is a text file with over 2MB in size so should pose no problem.
More to come....

Older HP enterprise series DCxxxxx i think with 2X AMD 2.5GHz processors.