Sigh,

Firstly, getting from uncompressed at the input to compressed is basic processing.

Seriously, you don't. Non risk instructions often take Multiple cycles, so you can only do a tiny number of them between samples, usually just enough to compress it to fit the bus speed without loss. https://en.m.wikipedia.org/wiki/Cycles_per_instruction

Thirdly, clearly you didn't rtfa.

Fourthly, go disagree on the teensy forum. https://forum.pjrc.com/threads/27364-Teensy-3-1-and-ADC-FIR-...