Status: iShowU Instant 2, builds 1.4.1 and later, are natively built for the new Mac M1.
And the performance is awesome. I have a more in depth answer below.
It can handle 60fps encoding of 6 megapixel frames (3880x1600) using just 50% CPU load. It doesnt actually feel like its doing anything at runtime. Quite remarkable.
Q: Is the app compatible with the new mac M1?
A. Yes. As of build 1.4.1, the app is built natively for the M1.
Q: How fast is it?
Performance exceeds that of an 2020 Intel iMac with AMD 5700 (my other development machine).
In testing; I had the app running up to 120fps internally. That's 120fps while capturing; compositing; and re-encoding (MUCH more that QuickTimeX is doing). All using 6MP frames. If I were capturing something smaller; theoretical performance would be much higher.
Q: What codec should I use? H264? HEVC?
I've left the default at H.264 for now. The app will auto-choose HEVC where necessary (super large captures). In my testing I saw HEVC perform marginally better (CPU less by 5% or so). There was no noticable impact on the "feel" while capturing.
The only reason I've left it at H264 is for compatibility.
Feel free to go HEVC on the M1. After all, it feels like you get it for free!