Just add to Mark's good explanation, there is also an adc with edma demo in MCUXpresso SDK.
The ADC conversion time is one aspect (as described) but the second aspect is using the data.
DMA can be used to save the samples to a circular buffer for subsequent firmware analysis or passing to other interfaces.
The uTasker project contains a turn-key reference that allows sampling an ADC input at > 400kHz to a circular buffer via DMA, transferring the signal to the DAC output (after a delay) and perform an FFT in the buffer. This shows that the K22F can achieve an FFT on a 200kHz bandwidth with frequency resolution of about 100Hz with a new result every 10ms using 50% of its CPU power.
There is also a reference via USB (which doesn't use the ADC but the principle is the same if USB is considered as a sampled digital analoge input) at https://www.youtube.com/watch?v=n-GABeILGV8&feature=youtu.be
Single-ended, lower resolution, fastest allowed ADC clock, no HW averaging and all settings set to minimum delays: Single conversion time around 1.24us / subsequent conversion times about 865ns