I looked at how USB was bit-banged on an arduino and I'm sure some of the problems involved are common with this. One constraint is VGA is at minimum around 25

I looked at how USB was bit-banged on an arduino and I'm sure some of the problems involved are common with this. One constraint is VGA is at minimum around 25mhz and HDMI is at minimum 165mhz and a pico is around 133mhz. It also takes clock cycles to update the data and run algorithms. Some of the algorithms can often be mitigated to the silicon that controls some of the pins as they can have small bit of programmable logic for this purpose or can be re-purposed, but usually not all the pins or all of the algorithms. There are tricks and such that might be able to get it working in certain lower resolutions and color modes, perhaps like overclocking or taking advantage of some harmonic, or clever use of some existing feature, but the more tricks you do to get things faster,often introduce issues with stability that you have to mitigate or solve. This barely touches physical constraints like cord length, signal integrity, voltages and current, overheating, etc that may or may not be applicable.