Sampling Frequency in DMA mode?

[mh-]

From the adc_timer_dma` example, I get the impression that I should use

  adc->adc0->startTimer(3000); //frequency in Hz

to set the sampling frequency, is this assumption correct?

Unfortunately, I cannot make sense of the results I see at the moment, I was trying e.g. 300000 and 1000000 (300kHz, 1MHz), but did not see an increase of 3.3 in the number of samples for a reference waveform (although it did slightly increase).
Am I hitting some limit?
I'm using

    adc->adc0->setAveraging(4); // set number of averages
    adc->adc0->setResolution(12); // set bits of resolution

on a Teensy 4.0

Thanks
Michael

P.S.: When changing from 30000 to 60000, I get twice the amount of samples, as expected. But from 60000 to 120000 nothing visible happens.

[mh-]

I found

    adc->adc0->setConversionSpeed(ADC_CONVERSION_SPEED::VERY_HIGH_SPEED); // change the conversion speed
    adc->adc0->setSamplingSpeed(ADC_SAMPLING_SPEED::VERY_HIGH_SPEED); // change the sampling speed

in this post: https://forum.pjrc.com/threads/66679-ADC-with-sampling-rate-of-4Msps-with-Teensy?p=274408&viewfull=1#post274408

...and it helps a lot.
Apparently without the VERY_HIGH_SPEED settings, the rate somehow limited; but using these additional settings, 300kHz works fine for my purposes.