The main issue isn't the CPU as such, it's memory. You can reasonably make a usable CPU using relays even, but a non-trivial amount of memory takes up sooo much space.
Ram takes so much space.
I remember a Minecraft mod from way back that let you do "modular" redstone, going "into" blocks to build redstone inside them, which you could then reuse as a single block. I wonder if it's been updated.