Did you actually look at that link to the Teensy processor boards https://www.pjrc.com/store/teensy31.html
The Teensy is a fully Arduino compatible board available in large quantities.
It hosts an Arm Cortex M4 processor running @ 96MHz. In fact by most standards it is a better Arduino board than any of the original Arduino boards. The Teensyduino plugin installs many libraries that work much better than the original libraries. The creator has 20+ years of professional experience as an embedded electronics developer and over the years has provided uncounted number of improvement to the original Arduino libraries.
For the price, size and performance it is a top notch product pretty much unmatched.