In the case of the Mini, the reference frequency is 24 MHz. To get 12MSPS, it’s a simple binary counter, but for 20MSPS, a whole PLL is run at a few hundreds of MHz, followed by many clock dividers. This adds noise and adds a few milliamps of current drain. The data throughput is also something to consider. More data means everything runs at a higher speed: ADC, DMA, USB transceiver, interrupts, etc. This all translates in more current and a higher thermal envelope.
