FWIW, a
Teensy 4.0 can sustain 25 Mbytes/sec over USB Serial, because it supports USB 2.0 High Speed (480 Mbit/s). (
Here is how you can verify it for yourself. It is not a synthetic benchmark, but a PRNG generator generating a pseudorandom sequence that passes all BigCrush tests (32 high bits of Xorshift64; "more random" than even Mersenne Twister), with a program on the computer receiving the output and verifying it matches a locally duplicated sequence, using a seed value the program running on the computer supplies. It does all that at a sustained data rate of 25 Mbytes/sec or more, even on my ye olde Core i5-7200U laptop.)
In other words, if you use DMA to obtain the ADC data, with a Teensy 4.x using bog-standard USB Serial you can sustain 25 Msps with 8-bit, 12 Msps with 16-bit, 8 Msps with 24-bit, or 6 Msps with 32-bit samples, and you only need to
Serial.write() in chunks of 32 to 1024 bytes, and only when
Serial.availableForWrite() reports your chunk size or a larger value, like shown
here.
There are some very cheap RISC-V -cored microcontrollers with high-speed USB 2.0 support, too, but I personally haven't verified what kind of performance one can get using something like the above test. I'm keeping my eye on CH32V307 and its extended family, though.