Raspberry Pi GPIO Performance? (I want to design a driver board)

Wouldn't using a realtime kernel help with that? I know it won't solve the issue, but it would mitigate it correct?

It's been a long time since I looked into linuxcnc, and back then, most people were using pentium 4 machines with a parallel port. Do people still use parallel ports a lot? Or has everyone moved on to dedicated step generation like with the mesa cards? I kinda want to land somewhere in between a parallel port and the Mesa cards in terms of performance with this driver board.

My experience with CNC is limited to 3d printers right now, so I'm pretty new to the higher precision when it comes to subtractive manufacturing. I have no idea what I should be shooting for right now. What would be an adequate step rate to shoot for?

(also, if this somehow comes off as argumentative or rude, please know that is not my intent. Just trying to learn)

Yes, you would be using a real time kernel (LinuxCNC depends on this)
The performance of software step generation depends on the real time capabilities
of this kernel.

I suspect that the RPI4 is capable of maybe 5 to 10 KHz software step rates
(depending on kernel, kernel options, whether the RPI is running video, etc etc)

This is adequate for a wide range of applications. The trade off is that your
maximum speeds at higher microstep ratios will be limited.

For example, a 5 KHz step rate with 1/4 stepping is 625 RPM, a respectable
speed for a step motor, but at a much smoother at 32 micros steps the maximum
RPM would only be a pokey 78 RPM.

Gotcha, that makes a ton of sense. So, if microstepping isn't a priority for someone, then a system doing software steps is adequate. Otherwise, a hardware step generator is critical.
I still want to make a board to accept the CM4, but now that I understand more of the limitations of software stepping, I'll definitely use a hardware step generator of some sort.

My goal is to have a 1MHz step rate per axis, and there's absolutely no way that's happening in software.

DIY 7i90SPI for Rpi is easy.

How easy is it ? Show me

PCW wrote: Yes, you would be using a real time kernel (LinuxCNC depends on this)
The performance of software step generation depends on the real time capabilities
of this kernel.

I think that someone was looking at using DMA to produce steps separately from the base thread some time ago. I don't know how far he got.