So, modern video cards include two devices - a video signal generator and a graphic accelerator.
The first of them converts data from video memory into MDA / CGA / VGA / RCA / DVI / HDMI / etc format.
The second is another co-processor (like mathematical co-processor), the purpose of which is to free the CPU from graphic calculations. This co-processor implements algorithms (as I understand it is not software but hardware) copying bits, drawing lines, scrolling, working with sprites, etc. It can implement acceleration of 2D or 3D calculations. Well, or both together.
Old computers, of course, had their own "video cards", or more precisely - video chips. If my searches are correct, not all of these "video cards" contained a video accelerator - that is, the poor CPU had to calculate the graphics completely by itself. And this includes computing graphics algorithms (Brezenham, etc.) and copying bytes. Least. For example, this is done in Atari 2600. This is a very slow way to work with graphics.
Sega Genesis, C64, Gameboy, Gameboy Color, Amiga's PC (just a couple of examples) had their own video chips for implementing graphics. Moreover, different platforms produced different graphics due to the different implementation of video chips.
So, since now I work with a homebrew for my pleasure, I can make design decisions that differ from those that were in old computers.
So, if in the system that I design, do not use video chips from old computers, but use another CPU. The program of which is independent of the main CPU and implements video algorithms - Bresenham's, scrolling, copying bytes, etc. These algorithms, of course, I should write. And the data exchange between CPU1 and CPU2 = GPU goes through a buffer. For example, CPU1 writes to the buffer that the GPU needs to draw a line from (x1, y1) to (x2, y2) or draw the desired sprite in the desired position. Well, this is an option, you can make an exchange in another way.
Will such a method be faster than classic old video chips?
For example, could Sega Genesis play heavier games if its video chip were replaced with a second Motorotola 68000 (8 Mhz) in the program of which video algorithms would be implemented?
(I understand that a direct replacement in the board will not work :) :)). I mean, if the engineers made that decision at the design stage.)