An RPi is not so much different from a PC in that the CPU core clock is going to be the fastest and everything else will be slower. You're not going to get 1.5GHz out of a GPIO.
https://raspberrypi.stackexchange.com/questions/100011/how-f...
You don’t need to. Your cap is 8MB/sec. For audio you don’t even need that. 48khz 16bit stereo is only 192k/sec.
I've seen some astounding projects using the Raspberry Pi Pico "PIO" to create n64 Flash carts and now HDMI adapters. I don't have the slightest knowledge of how this stuff works but I imagine it is some sort of bit banging like process. Do you think it would be possible to simulate something like a soundcard using Raspberry Pi Pico? These devices seem like a great and really cost effective way to design around expensive FPGA based designs.