How fast do you need it to run? I have one question: What clock frequency or specific performance features are you looking for in your fast AVR?

Most traditional AVRs limit the SPI clock in double‐speed mode to about fosc/2. That means if you run an AVR at 20 MHz, you’d typically get a maximum SPI clock of around 10 MHz. Could you please clarify if you need the microcontroller itself to run at 20 MHz (with the corresponding SPI max of ~10 MHz) or if you really require a SPI interface that can operate at 20 MHz? This will help narrow down the suitable parts.