how fast does the Teensy boot?
Teensy boots TOO FAST!
One of the common problems involves other chips that have brief start-up times. Motion sensors and Asian counterfeits of popular LED driver chips are the usual culprits. If you (or an Arduino library you use) makes a single attempt at startup to initialize the hardware, before it's booted up, the result can appear like Teensy isn't running your program. But in fact, your program is running and trying to make use of a chip that never got initialized, which is a case few people anticipate, so their programs usually "play dead" when the hardware isn't responding. The solution is usually just to add a delay at the beginning of setup(), before attempting to initialize other chips. Simple as that sounds, the problem usually manifests as Teensy appearing to not run your program, rather than an understanding that the boot time is too short.
But to answer your specific question, the boot time is approx 5 ms after the 3.3V power rises past approx 1.7 volts.
Most of that is an intentional 4 ms delay in the code, so you can delete that delay if you really want faster startup. Also consider any time spent in C++ constructors.
If you need fast startup, you can also put your own code into mk20dx128.c (or use one of the hooks provided in that code) to run functions very early in the boot process. The earliest one gives you control within just a few clock cycles of startup, but it's tough to use because almost nothing is set up... and the power supply isn't even fully settled and of course the CPU runs from a quick-starting internal oscillator. If you need *extremely* fast boot up, Teensy 3.2 can do it, within microseconds of the voltage rising to the point where the CPU can run.
But the normal ~5ms from power to setup() running is pretty fast, and in many cases so fast that conventional programs developed for slow-starting Arduino boards can have unexpected issues!